Setting the `FC` environment variable allows the user to override the
default Fortran compiler. The `SharedEnvExtension#fortran` method then
checks whether any of `FCFLAGS` and `FFLAGS` environment variables are
set and, if not, suggests to use `--default-fortran-flags` when running
`brew install`. However, `--default-fortran-flags` never makes it to
`build.rb` (that is executed by `FormulaInstaller`) because the flag is
not whitelisted in `FormulaInstaller#sanitized_ARGV_options`.
This change rectifies this and thereby eliminates the confusing message.
FixesHomebrew/homebrew#47852.
08fb36252c