refactor(test/mock-bin): extract generic environment variable echo option

[rami3l]

This PR is mainly cherry-picked from #5056 as suggested in #5056 (comment).

Also, since my project goal prototype branch will be modifying dist files a lot, I've used this opportunity to clean up the imports in dist as well.

[djc]

In rustls we enforce format style in CI by using nightly clippy:

https://github.com/rustls/rustls/blob/main/.github/workflows/build.yml#L392

Maybe that could make sense for rustup?

[rami3l]

In rustls we enforce format style in CI by using nightly clippy:

@djc If you mean nightly rustfmt, that'd be a more systematic solution indeed and I use something similar for my own projects. However it may break from time to time, probably due to other options I've been using at the same time.

Regarding the following minimal .rustfmt.unstable.toml that I propose below however, would it be stable enough according to your experience?

unstable_features = true

group_imports = "StdExternalCrate"
imports_granularity = "Crate"

[rami3l]

The key here is that we don't want the CI to break way too frequently which may slow down both merging and releases. But otherwise, having a consistent style everywhere would be super cool, I agree.

[djc]

group_imports = "StdExternalCrate"
imports_granularity = "Crate"

Yes, these have been very stable IME.

(I don't think you need unstable_features if you run nightly with these?)