workflows: Run conformance once a week (#1565)

This ensures there is always a result available for the client
conformance report (https://sigstore.github.io/sigstore-conformance/)

Signed-off-by: Jussi Kukkonen <jkukkonen@google.com>

Author: jku

.github/workflows/conformance.yml

@@ -6,6 +6,8 @@ on:
     branches: ['main']
   pull_request:
     branches: ['main']
+  schedule:
+    - cron: "45 7 * * 1"
 
 permissions:
   contents: read