Check for unexpected cargo/rustc before install

[brson]

There are already a number of pre-install sanity checks (in do_pre_install_sanity_checks) that there's no existing install. They don't catch every type of rust installation though.

Add another pre install check that iterates over the path looking for cargo and rustc, printing an error if it finds them, but only if they are not in the destination .cargo/bin, and only if the -y (no_prompt) flag is not set. The error should indicate that -y will override the error, like the anti_sudo check.

[brson]

I'd suggest doing this in a new method along side the other pre-flight checks. Test for this can go into cli-misc and will require creating a TempDir and populating it with cargo, adding it to PATH (tests run with a mutex so modifying the env is fine), and checking that rustup-init fails. Do it for both the cargo and rustc bins.