Use sys.executable when invoking python interpreter from tests
Use sys.executable when invoking python interpreter from tests

When we only have python3 installed, the test for missing argument is
failing because there is no "python" executable. Use `sys.executable`
instead. Also set environment correctly, like done in 7024d3b36176.

audreyr/cookiecutter,michaeljoseph/cookiecutter,pjbull/cookiecutter,audreyr/cookiecutter,luzfcb/cookiecutter,hackebrot/cookiecutter,hackebrot/cookiecutter,pjbull/cookiecutter,luzfcb/cookiecutter,michaeljoseph/cookiecutter