Bug 1692200 - Enclose path in single quotes to fix Windows mozbuild 3.3. breakage. r=IanN

suite/app/Makefile.in

@@ -101,7 +101,7 @@ pack-ext: $(STAGEDIST)
 	$(foreach dir,$(filter-out $(DONOTPACK),$(subst $(STAGEDIST)/,,$(wildcard $(STAGEDIST)/*))),$(_PACKAGE_EXTENSIONS))
 
 tools::
-	@$(MAKE) pack-ext STAGEDIST="$(DIST)/bin/extensions"
+	@$(MAKE) pack-ext STAGEDIST='$(DIST)/bin/extensions'
 
 # Additional macOS build steps.
 ifeq (cocoa,$(MOZ_WIDGET_TOOLKIT))