ci: cache .git dir in build-oci-image

.gitea/workflows/gentoo-utils.yml

@@ -11,6 +11,12 @@ jobs:
     runs-on: ubuntu-latest
     continue-on-error: true
     steps:
+      - name: Restore git cache
+        uses: actions/cache@v4
+        with:
+          path: .git
+          key: gitea-repo-${{ gitea.repository }}-${{ gitea.ref }}
+
       - name: Checkout repo
         uses: actions/checkout@v5
         with: