ignore user python packages by using `-s` option when running Python code via `python -c`

[Flamefire]

trafficstars

To avoid the user environment (Python packages installed to $HOME/lib/python*) pass -s to all invocations of python -c.

[boegel]

@Flamefire Also here, we should add a check to the test suite to avoid that use of python -c without -s is introduced again going forward...