fix(gtk): unset cache override during install

gtk/Makefile

@@ -30,7 +30,7 @@ install: $(PROG)
 	mkdir -p "$(DESTDIR)$$module_dir" && \
 	cp "$(PROG)" "$(DESTDIR)$$module_dir/" && \
 	if test -z "$(DESTDIR)"; then \
-		GTK_PATH= GTK_IM_MODULE_FILE= \
+		unset GTK_PATH GTK_IM_MODULE_FILE; \
 		$(GTK_QUERY_IMMODULES) --update-cache; \
 	fi
 
@@ -42,7 +42,7 @@ uninstall:
 	}; \
 	rm -f "$(DESTDIR)$$module_dir/$(PROG)" && \
 	if test -z "$(DESTDIR)"; then \
-		GTK_PATH= GTK_IM_MODULE_FILE= \
+		unset GTK_PATH GTK_IM_MODULE_FILE; \
 		$(GTK_QUERY_IMMODULES) --update-cache; \
 	fi