conformance: Schedule a weekly run

This way there is always an up-to-date result for the conformance
report (https://theupdateframework.github.io/tuf-conformance/)
to use

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

Author: jku

.github/workflows/conformance.yml

@@ -4,6 +4,8 @@ on:
       - develop
   pull_request:
   workflow_dispatch:
+  schedule:
+    - cron: '30 6 * * 3'
 
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   contents: read