Read target and Spack version from a config file

[CodeGat]

In #164 we are removing many unneeded ENV variables. $ENV_SPACK_ARCH is the only one referenced outside the build stage:

build-ci/.github/workflows/model-1-build.yml

Line 113 in 3c0840d

spack change --match-spec ${{ env.PACKAGE_NAME }} ${{ env.PACKAGE_NAME }}@git.$GH_REF%${{ matrix.compiler.COMPILER_NAME }}@${{ matrix.compiler.COMPILER_VERSION }} arch=$ENV_SPACK_ARCH

This is being changed in e39edcd to target, but we still need to keep track of it. @harshula noted that we should put it as a section in config/compilers.json as well.

Remove the ENV and instead propagate config/compilers.jsons .target to model-1-build.yml and other workflows.

From #164 (comment)