Option to only set the FC variable (or none)

[henryiii]

In GHA, it's impossible to unset a variable that's been set (actions/runner#1126). It would be nice to either be able to disable the non-FORTRAN variables (which tend to break Python extension compiling), or probably even better, since the paths are set as output variables, add a update-environment: false option that disables touching the environment altogether (matching actions/setup-python). Someone could use the output to set FC, so that option would be enough.

[wpbonelli]

@henryiii this is in the main branch now. I didn't add a test since it's a trivial change and our matrix is already too big. Let me know if it doesn't work for you.