Log Message:
py-google-cloud-sdk: Choose a safe fallback for Python interpreter

The logic to find a Python executable can be summarized as:
 - CLOUDSDK_PYTHON environment variable defined:
    + If it matches `*python2*', use it
 - Otherwise:
    + Try `python2'
    + Fallback to `python'

Substitute the latter fallback to ${PYTHONBIN} in order to have a
safe choice.

Should fix PR pkg/52122 reported by <jklos>.
This patch is based on the one proposed by <maya> (but restricted
to CLOUDSDK_PYTHON lines and the fallback `python').

Bump PKGREVISION.