eessi_container.sh script needs to take into account `$APPTAINER_BIND` if it is set

[migueldiascosta]

I just tried the eessi_container.sh script on a system configured to use apptainer and with $APPTAINER_BIND defined instead of $SINGULARITY_BIND, got WARNING: SINGULARITY_BIND and APPTAINER_BIND have different values, using the latter, and then the EESSI paths were not bind mounted

This is because the script defines $SINGULARITY_BIND without taking into account $APPTAINER_BIND, so then both of them are set and are different

https://github.com/EESSI/software-layer/blob/161a284b51f92709f925933cbd3326ee45f6ae96/eessi_container.sh#L822-L827

I simply replaced $SINGULARITY_BIND with $APPTAINER_BIND in the script and then it worked fine, but looks like more generically the script will need to take into account multiple scenarios