cmake: FindPythonInterp: Prioritize 'python' from path the highest

Users expect that not just "some" compatible python is detected, but
also that it will be the 'python' executable from PATH, at least when
this executable is valid.

To this end rewrite FindPythonInterp to give this executable the
highest priority.

This fixes #11857

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>