Rustfmt is not run if rustup exists, but nightly toonchain/rustfmt-nightly is not installed

[Kobata]

This code will attempt to run rustup run nightly rustfmt in any situation that rustup is available, without fallback to the non-nightly version in the event it fails, for instance due to there not being an installed nightly toolchain.

It appears to have been introduced by #1042

Obviously, there are some known issues with 'old' versions of rustfmt, so prioritizing nightly when possible isn't that bad an idea, however currently there is no fallback of any sort short of preventing bindgen from finding any rustup binary.

[Kobata]

rust-lang/rustup#1294 added a 'proxy' executable for the default path that will (once bundled rustfmt hits a channel) run the appropriate version for the default toolchain.

As such, it seems like the best option as of now is to remove the test for rustup and the alternate run entirely, and rely on whatever rustfmt is on the PATH: Anyone using nightly should (with minor setup) get the nightly version run, and once the new rustfmt hits stable, this test will not need updated to accomodate.