Singularity/Apptainer support

Adding initial support for Singularity/Apptainer. I have no idea if the final product of this works yet, will test next week.

.github/workflows/build_conda_recipes.yaml:
- I am first freeing up disk space since the Docker container + SIF image fill that up.
- Under the docker/build-push-action Action I use the load: true parameter to keep the Docker image on the machine, else it gets deleted.
- Under the Apptainer Build section, I first save the Docker image as a tar file, then I build a SIF file using apptainer build.
- I am then uploading the SIF file to the repo's GitHub Container Registry.
.github/workflows/apptainer.yaml:
- Keeping the apptainer chunk in this file for easier debugging.
Also see https://github.com/singularityhub/github-ci/issues/16#issuecomment-1177923705 for the bug in the GHCR interface.
And related to #218.

sigven / pcgr