GitHub Action CI: Add Go source file format check (#6090)

https://github.com/XTLS/Xray-core/pull/6057#issuecomment-4364819830

And https://github.com/XTLS/Xray-core/pull/6149#issuecomment-4546876261

.github/workflows/test.yml

@@ -1,4 +1,4 @@
-name: Test
+name: Tests and Checkings
 
 on:
   push:
@@ -8,6 +8,7 @@ on:
 jobs:
   check-assets:
     runs-on: ubuntu-latest
+    if: github.event_name != 'pull_request' || github.event.pull_request.head.repo.full_name != github.event.pull_request.base.repo.full_name
     steps:
       - name: Restore Geodat Cache
         uses: actions/cache/restore@v5
@@ -36,6 +37,7 @@ jobs:
 
   check-proto:
     runs-on: ubuntu-latest
+    if: github.event_name != 'pull_request' || github.event.pull_request.head.repo.full_name != github.event.pull_request.base.repo.full_name
     steps:
       - name: Checkout codebase
         uses: actions/checkout@v6
@@ -50,8 +52,28 @@ jobs:
             fi
           done
 
+  check-format:
+    runs-on: ubuntu-latest
+    if: github.event_name != 'pull_request' || github.event.pull_request.head.repo.full_name != github.event.pull_request.base.repo.full_name
+    permissions:
+      contents: read
+    steps:
+      - name: Checkout codebase
+        uses: actions/checkout@v6
+      - name: Set up Go
+        uses: actions/setup-go@v6
+        with:
+          go-version-file: go.mod
+          check-latest: true
+          cache: false
+      - name: Check Format
+        run: |
+          go install -v mvdan.cc/gofumpt@latest
+          go run ./infra/vformat/main.go -mode check -pwd ./
+
   test:
     needs: check-assets
+    if: github.event_name != 'pull_request' || github.event.pull_request.head.repo.full_name != github.event.pull_request.base.repo.full_name
     permissions:
       contents: read
     runs-on: ${{ matrix.os }}