Spack based workflow

[SeanBryan51]

Now that CABLE is installable with spack (see ACCESS-NRI/access-spack-packages#60), we can leverage features of spack in benchcab's workflow to build models in a way that:

1. Is model agnostic
2. Improves portability of benchcab to other machines
3. Makes the source code easier to maintain - using a package manager like spack to install different versions of CABLE is more modular than doing this manually. For example, moving to spack will simplify code and remove complex abstractions - resolving issues like:
   • Remove Repo hack from model.py #210
   • Unit tests for repo module #211
4. Allows for testing multiple model build configurations (e.g. different compilers, different compiler flags, different versions of dependencies, etc).