Allow deployment to multiple versions of Spack

[CodeGat]

For now, we shall deploy to v0.20 of spack. But, in future, we should make this a matrix deployment of spack versions, as well.

[CodeGat]

Except, past @CodeGat, maybe we should make it based on the model. We don't want to deploy to multiple versions of spack because we only really care about the end result.

[CodeGat]

In that case, this can be closed because each model has their own paths to a particular version of spack