Builder|qmake: Run Python 2 specifically

Python 3 is becoming commonplace as the default version, but our
scripts are still using Python 2. These changes ensure that
Python 2 (2.7) is run whenever Python is needed.

On Windows, however, it is up to the user to make sure that Python
2 is the default Python (on the PATH).
Unix|Builder|qmake: Use environment to find python