fix(CI): install python3 before artifacthub-pkg.yml validation

node:22-slim does not include Python. The validation step was failing
with "python3: not found" (exit 127) on every PR in every plugin repo.

Fix: add apt-get install step before the validation step.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.github/workflows/plugin-ci.yaml

@@ -19,6 +19,9 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v6
 
+      - name: Install Python
+        run: apt-get update && apt-get install -y --no-install-recommends python3 python3-yaml
+
       - name: Validate artifacthub-pkg.yml
         run: |
           python3 - <<'EOF'