extend/ENV/shared: use try.

`version_suffix` exists (or doesn't) depending on the GCC formula being
available and not loaded from the API.

Library/Homebrew/extend/ENV/shared.rb

@@ -289,7 +289,7 @@ module SharedEnvExtension
     gcc_version_name = "gcc@#{version}"
 
     gcc = Formulary.factory("gcc")
-    if gcc.version_suffix == version
+    if gcc.try(:version_suffix) == version
       gcc
     else
       Formulary.factory(gcc_version_name)