makefile: clean: run git clean as root (required to clean build files created by root)

Makefile

@@ -18,7 +18,7 @@ install_buildenv:
 # clear all caches, only required when changing the mirrors/architecture config
 clean:
 	sudo lb clean --all
-	git clean -di
+	sudo git clean -di
 
 bump_version:
 	@last_tag=$$(git tag | tail -n1); \