py-scikit-build-core should not pull in c, cxx, and fortran when not testing

[rbberger]

When installing on MacOS, we typically don't have a fortran compiler. Trying to install py-scikit-build-core leads to concretization errors because of this. Package installation succeeds even without any of these compilers, so it doesn't actually need any compilers unless run_tests is true.

So these dependencies could be marked type="test" only. However, because of how python sets up the dependent build environment for extensions, it needs compiler-wrapper to exist in the DAG, which apparently type="test" doesn't do. One solution could be a dedicated test variant.

@wdconinc Thoughts?