[lldb] Use shutil.which instead of distutils find_executable

distutils is deprecated and shutil.which is the suggested
replacement for this function.

https://peps.python.org/pep-0632/#migration-advice
https://docs.python.org/3/library/shutil.html#shutil.which

It was added in Python3.3 but given that we're already using
shutil.which elsewhere I think this is ok/no worse than before.

We do have our own find_executable in lldb/test/Shell/helper/build.py
but I'd rather leave that as is for now. Also we have our own versions
of which() but again, a change for another time.

This work is part of #54337.

Reviewed By: JDevlieghere

Differential Revision: https://reviews.llvm.org/D124601