Do not rebuild the build script unless necessary

[samueltardieu]

Without this guard, the build script is rebuilt as soon as any file within the package is changed. This includes files created by external tools such as IDE.

Fix #6640

[samueltardieu]

What should I do about the flaky tests (two of them being due to network issues)? Can someone rerun them?

[samueltardieu]

The failure on Windows seems irrelevant, but I'm not equipped to investigate:

> Run cargo nextest run --profile ci --hide-progress-bar --features=windows -p uucore -p coreutils
[…]
   Compiling proc-macro2 v1.0.86
error[E0463]: can't find crate for `profiler_builtins`
  |
  = note: the compiler may have been built without the profiler runtime

For more information about this error, try `rustc --explain E0463`.
error: could not compile `cfg-if` (lib) due to 1 previous error
warning: build failed, waiting for other jobs to finish...
error: could not compile `windows_x86_64_gnu` (build script) due to 1 previous error
error: could not compile `proc-macro2` (build script) due to 1 previous error
error: could not compile `version_check` (lib) due to 1 previous error
error: command `'\\?\C:\Users\runneradmin\.rustup\toolchains\nightly-x86_64-pc-windows-gnu\bin\cargo.exe' test --no-run --message-format json-render-diagnostics --package uucore --package coreutils --features windows` exited with code 101
Error: Process completed with exit code 1.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/inotify-dir-recreate (fails in this run but passes in the 'main' branch)

[sylvestre]

merci!