CI: replace actions/checkout with a shell git clone

The bare ubuntu:24.04 container has no Node.js, so the JS-based
actions/checkout@v4 fails with exit 127. Clone via plain git instead.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitea/workflows/build-flatpak.yml

@@ -30,8 +30,18 @@ jobs:
             ca-certificates curl git jq xz-utils zstd binutils \
             flatpak flatpak-builder
 
+      # Done as a plain git clone instead of actions/checkout@v4: the bare
+      # ubuntu image has no Node.js, so JavaScript actions fail with exit 127.
       - name: Checkout
-        uses: actions/checkout@v4
+        env:
+          TOKEN: ${{ secrets.PUBLISH_TOKEN != '' && secrets.PUBLISH_TOKEN || secrets.GITHUB_TOKEN }}
+        run: |
+          AUTH_URL="$(echo "${GITHUB_SERVER_URL}" | sed "s#://#://${GITHUB_ACTOR}:${TOKEN}@#")/${GITHUB_REPOSITORY}.git"
+          git init -q
+          git config --global --add safe.directory "$GITHUB_WORKSPACE"
+          git remote add origin "$AUTH_URL"
+          git fetch -q --depth 1 origin "${GITHUB_SHA:-$GITHUB_REF_NAME}"
+          git checkout -q FETCH_HEAD
 
       - name: Resolve latest O3DE version
         id: ver