Fix mantid launcher scripts

To test: For the workbenches (wherever you checked out)

./mantidworkbenchnightly --version
./mantidworkbenchqa --version
./mantidworkbench --version

For python

./mantidpythonnightly --classic -c "from mantid import __version__;print(__version__)"
./mantidpythonqa --classic -c "from mantid import __version__;print(__version__)"
./mantidpython --classic -c "from mantid import __version__;print(__version__)"

Refs https://code.ornl.gov/sns-hfir-scse/deployments/mantid-deploy/-/issues/2