Bug 1614136 - Clobber langpack output before packaging to avoid failures due to stale symlinks. r=nalexander

Differential Revision: https://phabricator.services.mozilla.com/D64411

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

toolkit/locales/l10n.mk

@@ -208,7 +208,7 @@ endif
 langpack-%: IS_LANGUAGE_REPACK=1
 langpack-%: IS_LANGPACK=1
 langpack-%: AB_CD=$*
-langpack-%:
+langpack-%: clobber-%
 	@echo 'Making langpack $(LANGPACK_FILE)'
 	@$(MAKE) libs-$(AB_CD)
 	@$(MAKE) package-langpack-$(AB_CD)