The `scikit` module is not available in `Stages/2022`. We instead add
the `scikit-learn` module, which actually has nothing to do with the
original one.

Improve everything

See merge request kesselheim1/sc_venv_template!1

ZSH does not set `BASH_SOURCE`, so we use a command to get the
corresponding path.

See also https://stackoverflow.com/a/28336473.

We now assign once at the top; this makes portability changes with
regard to this variable easier.

- more modern bash code (e.g. replace backtick expansions with
  `$([...])`)
- handle more paths (e.g. with spaces); this boils down to simply
  quoting every variable
- prefer clearer `${BASH_SOURCE[0]}` over `$BASH_SOURCE`

I picked the most recent versions that matched the newest stable GCC.
ParaStationMPI does not yet have GCC 11 modules and not sure about
stability of the GCC 11 module.

Better practice in order to separate the virtual environment cleanly
from the one it is started from.

This is simply clearer.

Could of course also use `./`, but then we may need to include `chmod
u+x` as well.