Makefile: ensure temp Go modules can be deleted

As a followup to commit 1d4e46b311a3 in PR #4132, we ensure
any files created during the "make test" build can be fully
deleted by setting the user write permission bit.  Otherwise,
at least on macOS, some Go modules' files are installed in
directories with read-only permissions, and the final "rm -fr"
cleanup is then unable to remove those files, leading to lots
of spurious error messages.

Makefile

@@ -473,6 +473,7 @@ test : fmt $(.DEFAULT_GOAL)
 		export GIT_CONFIG_NOSYSTEM=1; \
 		$(GO) test -count=1 $(GO_TEST_EXTRA_ARGS) $(addprefix ./,$(PKGS)); \
 		RET=$$?; \
+		chmod -R u+w "$$tempdir"; \
 		rm -fr "$$tempdir"; \
 		exit $$RET; \
 	)