Add -Zfmt-debug=none test in CI

[dtolnay]

Prior to #61, this would fail.

error: expected expression, found `<eof>`
 --> /home/runner/work/rustversion/rustversion/target/debug/build/rustversion-3dc54cd5d21b9264/out/version.expr:1:2
  |
1 |
  | ^ expected expression
error: non-expression macro in expression position: include
   --> src/lib.rs:217:30
    |
217 | const RUSTVERSION: Version = include!(concat!(env!("OUT_DIR"), "/version.expr"));
    |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
error: could not compile `rustversion` (lib) due to 2 previous errors