formula_installer: fix --default-fortran-flags

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.

Fixes Homebrew/homebrew#47852.

Library/Homebrew/formula_installer.rb

@@ -503,6 +503,7 @@ class FormulaInstaller
     args << "--verbose" if verbose?
     args << "--debug" if debug?
     args << "--cc=#{ARGV.cc}" if ARGV.cc
+    args << "--default-fortran-flags" if ARGV.include? "--default-fortran-flags"
 
     if ARGV.env
       args << "--env=#{ARGV.env}"