Have Action Verify Installed fpm Works

[everythingfunctional]

I think it would be desirable for this action to test that the fpm executable is also capable of building a simple project before completing. I recently had a situation where the CI environment changed such that the fpm executable no longer worked. See fortran-lang/fpm#780 . If the action had reported that the fpm executable didn't work, it would have been easier to diagnose the underlying problem.