ci: use gitea.com action mirrors instead of github.com

GitHub is returning 500 errors for actions/checkout and actions/setup-go.
Switch to Gitea's own mirrors at gitea.com/actions/ to avoid the dependency.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -10,8 +10,8 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
-      - uses: actions/setup-go@v5
+      - uses: https://gitea.com/actions/checkout@v4
+      - uses: https://gitea.com/actions/setup-go@v5
         with:
           go-version-file: go.mod
       - run: go build ./...
@@ -19,8 +19,8 @@ jobs:
   test:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
-      - uses: actions/setup-go@v5
+      - uses: https://gitea.com/actions/checkout@v4
+      - uses: https://gitea.com/actions/setup-go@v5
         with:
           go-version-file: go.mod
       - run: go test ./...
@@ -28,8 +28,8 @@ jobs:
   vet:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
-      - uses: actions/setup-go@v5
+      - uses: https://gitea.com/actions/checkout@v4
+      - uses: https://gitea.com/actions/setup-go@v5
         with:
           go-version-file: go.mod
       - run: go vet ./...