Add GitHub Workflow: Build model with and without optional submodules

This would require the use of a container, but: It would be good to check on every PR that the model can actually be built. #3214 happened because we weren't testing whether the model could build if the optional FTorch were checked out. We should check build success without optional submodules checked out too, though.

A basic implementation of this would test building a simple case, probably with both intel and gnu. We could add additional cases to improve coverage. At the extreme end, we could even test building all `aux_clm` and `ctsm_sci` tests (plus other suites?).