Add automated CI tests to validate generated pkg-config files

[srinjoy933]

Since our build system and the way we handle external dependencies like BLAS and LAPACK will likely keep evolving, I was thinking it might be a good idea to add a quick automated test to our CI pipeline to make sure these .pc files stay healthy.

We could add a simple check to our GitHub Actions workflows—maybe just in the Linux CMake build matrix. The idea would be to have it run right after the installation step. It would just need to make sure pkg-config is installed on the runner, point it to our new installation directory, and run the pkg-config --validate fortran_stdlib command. We could even have it test the flags to ensure everything resolves without errors.

I think this would be a really helpful way to catch any accidental regressions before they get merged.

[srinjoy933]

@jvdp1 @jalvesz what's your take on this idea?