Revert `-j 1` change in build.ps1 for foundation tests

[bnbarham]

Description

Running the build concurrently causes a failure during compilation. We believe this is likely some precondition, though haven't been able to actually confirm that with any output - it hasn't ever occurred with adding logging and there's no stacktrace/system log. Still actively looking into this, but filing so that we don't forget :)

Additional information

No response

[bnbarham]

I've tracked this down to swiftlang/swift-driver#1939. There's a bunch of problems here:

1. SwiftPM doesn't output the command line that failed. This makes sense normally, since we shouldn't spam with the command on a compilation failure. But it would be nice to have a --super-verbose that does output it.
2. swift-driver drops the error on the floor - ExecuteJobRule.executeJob returns a non-successful job completion on an error, but only outputs a diagnostic on if it is a DiagnosticData. That ends up throwing a Driver.ErrorDiagnostics.emitted (even though we haven't), which is caught in the top level (without any extra output).
3. Turns out atomic is ... not so atomic. See the test case below.

import Dispatch
import Foundation

let group = DispatchGroup()
let queue =
    DispatchQueue(label: "org.swift.sr83606.queue", attributes: .concurrent)

let path = URL(fileURLWithPath: "file.txt")
let contents = "text".data(using: .utf8)!

let count = Int(CommandLine.arguments.last ?? "1") ?? 1
for id in 0..<count {
  group.enter()
  queue.async {
    defer { group.leave() }
    do {
      try contents.write(to: path, options: [.atomic])
    } catch {
      print("Attempt \(id) failed: \(error)")
    }
  }
}
group.wait()

Just 2 causes this to fail most of the time. I haven't seen it not fail with 3.