fix: replace actions/checkout with git clone for host runner

The linux runner lacks node, so actions/checkout@v4 fails.
Use plain git clone instead.

.gitea/workflows/ci.yml

@@ -11,12 +11,14 @@ jobs:
     runs-on: linux
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        run: |
+          git clone --depth 1 --branch "${{ github.ref_name }}" \
+            "${{ github.server_url }}/${{ github.repository }}.git" workspace
 
       - name: Markdown lint
         run: |
           podman run --rm \
-            -v "${{ github.workspace }}:/work:Z" \
+            -v "$PWD/workspace:/work:Z" \
             docker.io/davidanson/markdownlint-cli2:v0.17.2 \
             "**/*.md"
 
@@ -24,12 +26,14 @@ jobs:
     runs-on: linux
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        run: |
+          git clone --depth 1 --branch "${{ github.ref_name }}" \
+            "${{ github.server_url }}/${{ github.repository }}.git" workspace
 
       - name: Check internal links
         run: |
           podman run --rm \
-            -v "${{ github.workspace }}:/work:Z" \
+            -v "$PWD/workspace:/work:Z" \
             -w /work \
             docker.io/library/python:3.12-slim \
             python3 scripts/check-links.py