macOS fixes, model curation, etc.

[olupton]

• fix --virtual on macOS
• support fetching models from forked repositories on GitHub
• disable build parallelism internal to nrnivmodl
• do some more model curation

[olupton]

Launched https://github.com/neuronsimulator/nrn-modeldb-ci/actions/runs/4281622878 with all models.

[olupton]

Launched https://github.com/neuronsimulator/nrn-modeldb-ci/actions/runs/4282403327 with b5ca035 and all models.

[olupton]

Launched https://github.com/neuronsimulator/nrn-modeldb-ci/actions/runs/4282403327 with b5ca035 and all models.

This doesn't yet show a trivial diff, but I will merge this PR as it is already approved/reviewed and is a step in the right direction.