[singularity] provide opt-out for tmp mount

[mlin]

https://github.com/chanzuckerberg/miniwdl/blob/main/WDL/runtime/backend/singularity.py#L94-L105

The default singularity configuration makes very little scratch disk space available, so we mount /tmp and /var/tmp to locations inside the run directory. This provides a good out-of-the-box experience, but might be undesirable in certain situations (e.g. https://github.com/miniwdl-ext/miniwdl-slurm/issues/6). Add a config options so that users can opt-out of the tmp mount (assuming they're able to reconfigure singularity to provide more scratch space).