Top level symlinks cannot be replicated inside the container

Singularity always follows symlinks provided as arguments to bind mounts, so there is currently no known way
to replicate e.g `/appl -> /pfs/lustrep3/appl` inside the container mount namespace.

This leads to issues when doing nested calls to the wrappers.

Quick fix is to call the `_bin` folder which will contain symlinks if the `bin` <-> `_bin` bind mount has not been applied. or then alternatively using the `--writable-tmpfs` and create a symlink at container startup.