Add nightly rustfmt to workflows

[WillLillis]

I noticed that invoking rustfmt from within my editor works as expected, but running cargo fmt from the command line leads to the following warnings:

Warning: can't set `wrap_comments = true`, unstable features are only available in nightly channel.
Warning: can't set `format_code_in_doc_comments = true`, unstable features are only available in nightly channel.
Warning: can't set `comment_width = 100`, unstable features are only available in nightly channel.
Warning: can't set `format_macro_matchers = true`, unstable features are only available in nightly channel.
....
Warning: can't set `format_macro_matchers = true`, unstable features are only available in nightly channel.
Warning: can't set `imports_granularity = Crate`, unstable features are only available in nightly channel.
Warning: can't set `group_imports = StdExternalCrate`, unstable features are only available in nightly channel.

Running cargo +nightly fmt of course fixes this locally.

I then noticed that these same warnings appear on the CI runs, for example in the "Run make lint" step of this workflow run. I did some testing within my fork and found that adding the changes in this PR fixes the issue, but I'm also not super familiar with Github workflows so I could also be missing something.