Pull arch information out of the Dockerfiles and into the base-spack matrix

[CodeGat]

We might want to test various architectures with our CI rather than just linux-rocky8-x86_64. Since this information is used in both base-spack (for installing the compiler) and build-<model> (for installing the packages), it must be done early in the matrix strategy.
Potentially in a [[compiler x arch] x model] format. So:

--- build-and-push-image-build ------ base-spack (comp1, linux-rocky8-x86_64) ------- build (access-om2, comp1, linux-rocky-x86_64)
                                 |                                              | --- build (access-om3, comp1, linux-rocky-x86_64)
                                 |--- base-spack (comp1, windows-10-x86_64) --------- build (access-om2, comp1, windows-10-x86_64)
                                 |                                              | --- build (access-om3, comp1, windows-10-x86_64)
                                 |--- base-spack (comp2, linux-rocky8-x86_64) ------- build (access-om2, comp2,  linux-rocky8-x86_64)
                                 | ...

Luckily, the arch information will be stored in an ENV in the base-spack image, so it is easily accessible via SPACK_ENV_ARCH.

[CodeGat]

Deprioritizing this issue as the need for different architectures isn't pressing. Can revisit later.