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Hello, it's me again on behalf of the rustup team here!

We have noticed a common issue with a regular brew user developing with Rust:

1. They often start with a perfectly fine rustup installation (either installed with or without brew).
2. After issuing brew install rust (usually implicitly as a build dependency), the binaries provided by that package will shadow existing rustup-provided binaries.
3. The existing rustup-installed toolchains/components might seem "gone", causing subtle breakages that are essentially out of our control.

We already have warnings when brew install rust happens before rustup is installed, but unfortunately checking the existence of a brew-managed Rust installation will be much more costly afterwards, so I'm making this PR hoping to further inform potential users of this conflict in the "after" case.

Hopefully closes rust-lang/rustup#1236.