Since we just tried running `rustup component add`, it doesn't make sense to me
to recommend trying that again. If we're reaching this case, it's probably more
likely that rustc was installed via package manager, in which case the source
should be installed the same way (e.g. if you install the rust-src package on
Ubuntu it will install a symlink in the right place to make our sysroot
detection work).
Folks report a ton of hard-to-diagnose issues, the solution for which
is "unset RUST_SRC_PATH". Let's just ignore RUST_SRC_PATH when it
won't work anyway!
