Bug 1616630 - Add PYTHON3 to fastermake variables; r=firefox-build-system-reviewers,rstewart

Without PYTHON3 defined, we can't actually run any GENERATED_FILES
scripts in the fastermake backend.

Differential Revision: https://phabricator.services.mozilla.com/D63437

--HG--
extra : moz-landing-system : lando

python/mozbuild/mozbuild/backend/fastermake.py

@@ -169,6 +169,7 @@ class FasterMakeBackend(MakeBackend, PartialBackend):
         # Add a few necessary variables inherited from configure
         for var in (
             'PYTHON',
+            'PYTHON3',
             'ACDEFINES',
             'MOZ_BUILD_APP',
             'MOZ_WIDGET_TOOLKIT',