fix: resolve Go vet module recognition issue in CI

- Remove redundant GO111MODULE environment variable (default in Go 1.16+)
- Add cache: true to setup-go action for better performance
- Ensure go mod tidy and verify run before go vet
- Simplify workflow structure to match best practices

.gitea/workflows/ci.yml

@@ -70,28 +70,23 @@ jobs:
         uses: actions/setup-go@v5
         with:
           go-version: '1.25.3'
-      - name: Download dependencies
+          cache: true
+      - name: Install dependencies
         working-directory: bugulma/backend
-        env:
-          GO111MODULE: on
         run: go mod download
-      - name: Vet
+      - name: Tidy and verify modules
         working-directory: bugulma/backend
-        env:
-          GO111MODULE: on
         run: |
           go mod tidy
           go mod verify
-          go vet ./...
+      - name: Vet
+        working-directory: bugulma/backend
+        run: go vet ./...
       - name: Test
         working-directory: bugulma/backend
-        env:
-          GO111MODULE: on
         run: go test -v -race -coverprofile=coverage.out ./...
       - name: Coverage
         working-directory: bugulma/backend
-        env:
-          GO111MODULE: on
         run: go tool cover -html=coverage.out -o coverage.html
 
   backend-build: