[Feat] --python flag for grass79.py

[HuidaeCho]

Is your feature request related to a problem? Please describe.

In case there are multiple installations of Python, it would be great to be able to execute the version associated with the GRASS_PYTHON environment variable for batch jobs.

Describe the solution you'd like

It can be either --exec python (a special case of --exec; if the batch job command is python, run GRASS_PYTHON instead of one in PATH; it can be a surprise though if the user intended to use non-GRASS_PYTHON in PATH, but why?) without adding a new flag, or --python path/to/script.py args... (better? but yet another flag).

Describe alternatives you've considered

Just leave it to the user. --exec /path/to/grass/python3 script.py args... with no changes.

Additional context

I encountered this issue while writing the workflow script for OSGeo4W (#692). There were two python installations from OSGeo4W (GRASS_PYTHON) and MSYS2, but MSYS2's path was in front of OSGeo4W by default. I had to specify the full path to the OSGeo4W python.