chore(ci): check rustfmt with nightly toolchain

[elenaf9]

Description

Some of our rustfmt rules aren't stable yet. Thus our rustfmt CI check should run on nightly.

Notes & open questions

Depends on #5742.

I am not super familiar with CI workflows. Can/ should this be solved differently?

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• A changelog entry has been made in the appropriate crates

[kamuik16]

run: cargo fmt -- --check -> run: cargo +nightly fmt -- --check I think.

[elenaf9]

run: cargo fmt -- --check -> run: cargo +nightly fmt -- --check I think.

Isn't this implicitly done because we directly install the nightly toolchain? The rustfmt job on this PR fails as expected on master with the current change: https://github.com/libp2p/rust-libp2p/actions/runs/12329671999/job/34414253861?pr=5743.

[kamuik16]

run: cargo fmt -- --check -> run: cargo +nightly fmt -- --check I think.

Isn't this implicitly done because we directly install the nightly toolchain? The rustfmt job on this PR fails as expected on master with the current change: https://github.com/libp2p/rust-libp2p/actions/runs/12329671999/job/34414253861?pr=5743.

not that I know of, but I could be wrong.

[jxs]

Thanks for going ahead with this Elena, I noticed when I ran locally tat there were files needing fmt.
We just need to run cargo fmt now so for the CI to pass