ENV: avoid misleading Fortran setup warnings

Fixes Homebrew/homebrew#31156.
Closes Homebrew/homebrew#47246.

Signed-off-by: Martin Afanasjew <martin@afanasjew.de>

Library/Homebrew/extend/ENV/shared.rb

@@ -203,6 +203,12 @@ module SharedEnvExtension
   end
 
   def fortran
+    # Ignore repeated calls to this function as it will misleadingly warn about
+    # building with an alternative Fortran compiler without optimization flags,
+    # despite it often being the Homebrew-provided one set up in the first call.
+    return if @fortran_setup_done
+    @fortran_setup_done = true
+
     flags = []
 
     if fc