Support envrc for rustfmt

[amesgen]

I don't have rustfmt installed globally, but rather get it per-project via Nix + direnv.

A more principled solution would be inheritenv, but I think this is simple enough.

Similar to #447.

Less noisy diff due to difftastic:

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @brotzeit (or someone else) soon.

Please see the contribution instructions for more information.

[brotzeit]

Sorry for the delay.