Warning on MacOS when installing the same version of gfortran as on the runner

[rikardn]

The following warning is emitted by brew when trying to install the same version of gfortran (gcc) that is already installed on the runner.

Warning: gcc@12 12.4.0 is already installed and up-to-date.
To reinstall 12.4.0, run:
  brew reinstall gcc@12

This is a minor thing, but since github collects all the warnings in the summary page after a run this adds to the noise.

It should be possible to turn off all warnings from brew with -q, but this might not be desirable in general.

[wpbonelli]

I don't think we want to turn off all warnings. We could check if the same gcc is already installed in install_gcc_brew like we do in install_gcc_apt and install_gcc_choco (in setup-fortran.sh).