fix: use forgejo runner to validate actions

Co-authored-by: Copilot <copilot@github.com>

.forgejo/workflows/validate-shared-actions.yml

@@ -9,15 +9,12 @@ permissions:
 
 jobs:
   validate-shared-actions:
-    runs-on: ubuntu-latest
+    runs-on: stackit-ubuntu-20
     steps:
       - name: Checkout
         uses: actions/checkout@v4
-
       - name: Validate shared action metadata
-        uses: mpalmer/action-validator@v0.9.0
+        uses: docker://data.forgejo.org/forgejo/runner:12
         with:
-          version: 0.9.0
-          patterns: |
-            :(glob)**/action.yml
-            :(glob)**/action.yaml
+          entrypoint: /bin/sh
+          args: -ec "find . -mindepth 2 -maxdepth 2 -name action.yml -exec forgejo-runner validate --action --path {} \\;"