Unset `PYTHONNOUSERSITE` in environment before calling batch script (#3313)
Unset `PYTHONNOUSERSITE` in environment before calling batch script (#3313)

This lets us have PYTHONNOUSERSITE=True for SCT scripts, while also restoring the original state of the environment for `sct_run_batch` shell scripts. 

(This is mainly for the `gradunwarp` package, which can rely on user site packages.)
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox