add comment about missing frontend build

Author: silverwind

.github/workflows/pull-compliance.yml

@@ -95,6 +95,8 @@ jobs:
         with:
           go-version: ">=1.20"
           check-latest: true
+      # no frontend build here as backend should be able to build
+      # even without any frontend files
       - run: make deps-backend deps-tools
       - run: go build -o gitea_no_gcc # test if build succeeds without the sqlite tag
       - name: build-backend-arm64