Update from singularity to apptainer

[nmerket]

Related to #170

Singularity has been renamed to Apptainer and is a couple of versions ahead of the version we have pinned in this repo. NREL's HPCs are using apptainer now, so we should upgrade.

We have a pretty convoluted process for building the apptainer image:

1. Building singularity from source inside a docker container.
2. Building the Singularity inside Docker.

It looks like with more modern apptainer, this could be much simplified. They have prebuilt apptainer packages for Ubuntu, which could be directly installed on the runner. Then the process of building an Apptainer image from a docker image is pretty straightforward:

apptainer build openstudio-3.7.0-rc2.sif docker://nrel/openstudio:3.7.0-rc2

[nmerket]

It looks like there's an existing setup-apptainer action. It's been a while since it has been updated, but could be an easy button option.