files/toolchain-shar-extract.sh: Rework PATH cleaning

Trying to create a clean PATH breaks cases where we install a buildtools tarball
on hosts to provide newer versions of gcc. Rework the fix for #8698 to clean up
directories in PATH which don't exist isntead. Do it with python as the shell
version was too fraught with corner cases.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>