gce: support virtualenv by detecting python via /usr/bin/env

This is likely also required for anybody who needs to install a different python to that provided by their system.

There are almost 160 other similar shebangs in the project, if somebody thinks all of these need to be fixed:

    git grep -E '^#!' |egrep -v 'make|env' | wc -l

FWIW as a non-python user it took me ~ 4h to figure out why the scripts failed and yet an interactive python session would succeed. Worth fixing!