[ogs] Relax embedded Python venv site-packages validation

venv detection during runtime relied on executing the python executable and then comparing python's version with the ogs embedded interpreter version. When running ogs via a (Guix-created) container, (usually) there is no python in the container and then the hosts python executable is tried to be executed which fails because of missing dependencies. Now the relaxed version of the check is done on paths of the site-packages directory only (on Linux and macOS, on Windows old behaviour is used). Should work for all use cases.

Also added python (the executable) and pip to the container.

• Add this merge request to the changelog