Remove LayrLab's docker image, use step-security's app

Author: aehyvari

.github/workflows/certora-prover.yml

@@ -36,13 +36,17 @@ jobs:
        startsWith(github.ref, 'refs/heads/certora/')) ||
       github.event_name == 'schedule' ||
       github.event_name == 'workflow_dispatch'
-    runs-on: protocol-x64-16core
+    runs-on: ubuntu-latest
+#    runs-on: protocol-x64-16core
     permissions:
       contents: read
       statuses: write
       pull-requests: write
       id-token: write
     steps:
+      - uses: step-security/harden-runner@ec9f2d5744a09debf3a187a3f4f675c53b671911
+        with:
+          egress-policy: audit
       - name: Checkout repository
         uses: actions/checkout@v4
         with: