ci: Run rustfmt on x86_64-linux only

[tesuji]

Also removing obsolete PATH="$HOME/rust/bin:$PATH".

[kinnison]

Does this honestly save a lot of CI time? I'd rather we ran cargo fmt everywhere just in case it catches something that conditional compilation made hard for it to spot unless it's consuming a huge quantity of time.

It takes less than a second to check on my slowest computer for example, so even if it took 5s on each of our platforms, that's hardly a lot of time.

What is your justification for this change?

[tesuji]

I would say this doesn't save noticeable build time. The rustfmt
on Linux builds are same program (same executable, same architecture).
So I personally don't want to run it multiple times. I agree with you on
keeping rustfmt running on Windows.

[kinnison]

Okay, so perhaps one rustfmt on linux, one on macos, and one on windows?