MAR-234: Use the pipreqcheck user when checking the Python version and prompt:

- Using the root account can result in incorrect permissions being set
on the Python cache (__pycache__) directories, which can further
cause permission issues for the pirpeqcheck user itself.
- The prompt also makes more sense to check in context of the virtual
environment user (since that's the one that will normally get used
with the virtual environment itself).