Remove default stable install on Windows

[Osspial]

Right now, even though we call rustup set profile minimal, Windows still downloads and installs the rust-docs component. This is because GitHub's default rustup install comes with the stable toolchain installed under the default profile, which contains rust-docs, resulting in rustup updating the rust-docs component despite our request otherwise. This PR removes the default stable Rust install, speeding up the Rust install process from about 3 minutes to 30 seconds.

Note that this PR simply moves the rustup toolchain folder to a new location, rather than calling rustup toolchain uninstall stable. This is because the official uninstall method removes all the rust documentation files from the filesystem, which takes a while. Using that rather than the method presented here takes 2 minutes, which is still faster than the status quo but significantly slower than this PR.

[hecrj]

Thanks @Osspial!

Does the issue you describe also affect Ubuntu? Maybe we should implement a similar fix for it too.

[Osspial]

It looks like Ubuntu also has rust-docs installed by default, but it's significantly less of an issue there because rust-docs takes much less time to manage. The solution here should largely work for Ubuntu, except you'd need to use whereis instead of where to get the .cargo path.