run_tests: fix pytest selection inside tox venv

Finding the "right" pytest is challenging.  In Debian, `pytest` is
Python 2 while `pytest-3` is the Python 3 version ... but only when
outside of a virtualenv.  Inside of a virtualenv (e.g. the ones that
tox creates), we always want `pytest`.

Change-Id: Ic1fe84c10f06227bceeb9baad6a3c4598bbe9860
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/303802


Reviewed-by: Peter Kjellerstedt <peter.kjellerstedt@gmail.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Mike Frysinger <vapier@google.com>