add sigstore prober (#224)

Signed-off-by: Brian DeHamer <bdehamer@github.com>

.github/workflows/prober-github.yml

@@ -0,0 +1,17 @@
+name: GitHub Sigstore Prober
+
+on:
+  workflow_dispatch:
+  schedule:
+    # run every 5 minutes, as often as Github Actions allows
+    - cron: '*/5 * * * *'
+
+jobs:
+  prober:
+    permissions:
+      attestations: write
+      id-token: write
+    secrets: inherit
+    uses: ./.github/workflows/prober.yml
+    with:
+      sigstore: github