[BUG] Can't use with rustfmt installed not by rustup

[CoelacanthusHex]

After #45, cargo-expand only checks rustfmt installed by rustup.
But many people, install rustfmt from the distro package manager, such as the rust package of Arch (include rustfmt).
In this case, cargo can't find rustfmt, then aborted and told user they must install rustfmt with rustup.
It's a REALLY REALLY TERRIBLE idea because it restricts users' freedom to choose their own tool installation method.
Hopefully, you can seriously consider whether you must use rustup.

[dtolnay]

I think the implementation in this crate is fine.

If someone is interested in the toolchain_find crate supporting non-rustup environments, that should be made as a PR to the toolchain_find crate.