feat: nixos installed rustfmt basic support

[dzmitry-lahoda]

I use nix to install rustfmt,

so assume to support default installed rustfmt can just try to search path for rustfmt binary.

So

rustfmt-wrapper/src/lib.rs

Line 105 in bc2effe

fn which_rustfmt() -> Option<PathBuf> {

will be

1. env
2. rustup
3. PATH

search

[ahl]

Would welcome a PR.

[spikespaz]

Came here planning to open a PR looking for prior art. I'm wondering why we don't use which (there's a crate) to find rustfmt as a fallback.