make: refactor build target into per-module dependencies

- Replace shell loop in build with explicit module targets
- Add .PHONY for build and module targets
- Keep go build -C <module> -buildmode=pie behavior unchanged
- Enable parallel builds via make -jN build

Makefile

@@ -1,4 +1,8 @@
 MODULES := devel-importer utils/hujson utils/maintainer-update gitea-events-rabbitmq-publisher gitea_status_proxy group-review obs-forward-bot obs-staging-bot obs-status-service workflow-direct workflow-pr
 
-build:
-	for m in $(MODULES); do go build -C $$m -buildmode=pie || exit 1 ; done
+.PHONY: build $(MODULES)
+
+build: $(MODULES)
+
+$(MODULES):
+	go build -C $@ -buildmode=pie