Sanity check for proxies created during the installation phase

[rami3l]

As suggested by @ChrisDenton:

Previous issues such as #4291 proved that the symlink approach to proxies on Windows could be problematic from time to time.

Maybe we could add a sanity check right after the installation phase for the proxie(s) with a backing binary, say running cargo --version or rustc --version, and see if the proxy can actually work and produce a reasonable output. If it doesn't, we should prompt the user to file an issue in https://github.com/rust-lang/rustup/issues and exit immediately with an error.

Note

• It's unfortunately not possible to assume any backing binaries to be available at all times. Not all toolchains have cargo, and rustup-init --default-toolchain=none can even leave you without rustc.
• For packagers who would like to skip the installation phase entirely, the validation verification of proxies should not be our responsibility.

[manyinsects]

@rustbot claim