Use xcrun -n to avoid (spotlight?) caching

This has lead to mysterious infinit-loop-bugs and people report
xcrun and ruby play ping-pong for ever (no one has actually waited
that long, but approximately).

Adding -n seaches the Xcode-included bin and xctoolchain dirs (even
if they are not in the PATH) and all dirs in the PATH. That is what
we wanted from the beginning.

The first call to `xcrun` with `-n` is about one second slower but
Homebrew caches these lookups anyway.

Related issues: #17426, #14691