Workflows: back to actions/checkout, on v5

The v4 pin was the problem, not the action; use the current major.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -12,16 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     container: rust:1
     steps:
-      # Plain git instead of actions/checkout: no dependence on the
-      # runner resolving external actions or injecting node into the
-      # container. Works for private repos via the workflow token.
-      - name: checkout
-        run: |
-          git init -q .
-          git fetch --depth 1 \
-            "https://gitea:${{ secrets.GITHUB_TOKEN }}@${GITHUB_SERVER_URL#https://}/${GITHUB_REPOSITORY}.git" \
-            "$GITHUB_SHA"
-          git checkout -q FETCH_HEAD
+      - uses: actions/checkout@v5
       - name: rustfmt
         run: |
           rustup component add rustfmt