Don't generate bootstrap usage unless it's needed

Previously, `x.py` would unconditionally run `x.py build` to get the
help message. After #76165,
when checking the CI stage was moved into `Config`, that would cause an
assertion failure (but only only in CI!):

```
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `1`,
 right: `2`', src/bootstrap/config.rs:619:49
```

This changes bootstrap to only generate a help message when it needs
to (when someone passes `--help`).

src/bootstrap/flags.rs

@@ -98,7 +98,6 @@ impl Default for Subcommand {
 
 impl Flags {
     pub fn parse(args: &[String]) -> Flags {
-        let mut extra_help = String::new();
         let mut subcommand_help = String::from(
             "\
 Usage: x.py <subcommand> [options] [<paths>...]
@@ -170,16 +169,6 @@ To learn more about a subcommand, run `./x.py <subcommand> -h`",
             "VALUE",
         );
 
-        // fn usage()
-        let usage =
-            |exit_code: i32, opts: &Options, subcommand_help: &str, extra_help: &str| -> ! {
-                println!("{}", opts.usage(subcommand_help));
-                if !extra_help.is_empty() {
-                    println!("{}", extra_help);
-                }
-                process::exit(exit_code);
-            };
-
         // We can't use getopt to parse the options until we have completed specifying which
         // options are valid, but under the current implementation, some options are conditional on
         // the subcommand. Therefore we must manually identify the subcommand first, so that we can
@@ -263,12 +252,38 @@ To learn more about a subcommand, run `./x.py <subcommand> -h`",
             _ => {}
         };
 
+        // fn usage()
+        let usage = |exit_code: i32, opts: &Options, verbose: bool, subcommand_help: &str| -> ! {
+            let mut extra_help = String::new();
+
+            // All subcommands except `clean` can have an optional "Available paths" section
+            if verbose {
+                let config = Config::parse(&["build".to_string()]);
+                let build = Build::new(config);
+
+                let maybe_rules_help = Builder::get_help(&build, subcommand.as_str());
+                extra_help.push_str(maybe_rules_help.unwrap_or_default().as_str());
+            } else if !(subcommand.as_str() == "clean" || subcommand.as_str() == "fmt") {
+                extra_help.push_str(
+                    format!("Run `./x.py {} -h -v` to see a list of available paths.", subcommand)
+                        .as_str(),
+                );
+            }
+
+            println!("{}", opts.usage(subcommand_help));
+            if !extra_help.is_empty() {
+                println!("{}", extra_help);
+            }
+            process::exit(exit_code);
+        };
+
         // Done specifying what options are possible, so do the getopts parsing
         let matches = opts.parse(&args[..]).unwrap_or_else(|e| {
             // Invalid argument/option format
             println!("\n{}\n", e);
-            usage(1, &opts, &subcommand_help, &extra_help);
+            usage(1, &opts, false, &subcommand_help);
         });
+
         // Extra sanity check to make sure we didn't hit this crazy corner case:
         //
         //     ./x.py --frobulate clean build
@@ -436,24 +451,11 @@ Arguments:
         let paths = matches.free[1..].iter().map(|p| p.into()).collect::<Vec<PathBuf>>();
 
         let cfg_file = env::var_os("BOOTSTRAP_CONFIG").map(PathBuf::from);
-
-        // All subcommands except `clean` can have an optional "Available paths" section
-        if matches.opt_present("verbose") {
-            let config = Config::parse(&["build".to_string()]);
-            let build = Build::new(config);
-
-            let maybe_rules_help = Builder::get_help(&build, subcommand.as_str());
-            extra_help.push_str(maybe_rules_help.unwrap_or_default().as_str());
-        } else if !(subcommand.as_str() == "clean" || subcommand.as_str() == "fmt") {
-            extra_help.push_str(
-                format!("Run `./x.py {} -h -v` to see a list of available paths.", subcommand)
-                    .as_str(),
-            );
-        }
+        let verbose = matches.opt_present("verbose");
 
         // User passed in -h/--help?
         if matches.opt_present("help") {
-            usage(0, &opts, &subcommand_help, &extra_help);
+            usage(0, &opts, verbose, &subcommand_help);
         }
 
         let cmd = match subcommand.as_str() {
@@ -483,7 +485,7 @@ Arguments:
             "clean" => {
                 if !paths.is_empty() {
                     println!("\nclean does not take a path argument\n");
-                    usage(1, &opts, &subcommand_help, &extra_help);
+                    usage(1, &opts, verbose, &subcommand_help);
                 }
 
                 Subcommand::Clean { all: matches.opt_present("all") }
@@ -494,12 +496,12 @@ Arguments:
             "run" | "r" => {
                 if paths.is_empty() {
                     println!("\nrun requires at least a path!\n");
-                    usage(1, &opts, &subcommand_help, &extra_help);
+                    usage(1, &opts, verbose, &subcommand_help);
                 }
                 Subcommand::Run { paths }
             }
             _ => {
-                usage(1, &opts, &subcommand_help, &extra_help);
+                usage(1, &opts, verbose, &subcommand_help);
             }
         };