Suggestion: use setup-fortran action from Marketplace

[spco]

Using https://github.com/fortran-lang/setup-fortran might mean we can slim down our own hand-crafted Fortran compiler install code. I've not looked over it at all to check it would work, but looks good.

It would help with moving to ifx support a la #510 , although looking at the matrix, macos+ifx is not yet supported in the Action.