batchspawner-singleuser prevents usage of off-the-shelf jupyter containers

### Bug description

Since the advent of the wrapper script `batchspawner-singleuser` we can not offer our HPC users to just use off-the-shelf Jupyter container images anymore (e.g. jupyter/datascience-notebook). Instead, the containers do need to contain an installed batchspawner package, which is usually never the case if downloaded from e.g. dockerhub, nvcr.io, ....
When JupyterLab stacks are loaded via Lmod (non-containerized), these stacks also need to have batchspawner to be installed.

#### Expected behaviour

The old version of batchspawner (0.8.1) allowed to provide JupyterHub + Slurmspawner + off-the-shelf JupyterLab inside docker containers (enroot runtime).

#### Actual behaviour

Things like this happen:
```
/var/spool/slurmd/job18872/slurm_script: line 30: batchspawner-singleuser: command not found
```

### Your personal set up

We use a regularly installed JupyterHub + batchspawner in conjunction with JupyterLab in Docker containers using [enroot](https://github.com/NVIDIA/enroot) runtime. The integration of enroot and slurm ist done via [pyxis](https://github.com/NVIDIA/pyxis).

**How to revert to the state as of 0.8.1 that no batchspawner is required for spawning of the (containerized or not) JupyterLab stacks?**