workflows: Enable tuf-conformance for PRs

tuf-conformance workflow now pins a release tag so we can enable this
on PRs.

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

Author: jku

.github/workflows/conformance.yml

@@ -1,5 +1,8 @@
 on:
-  # manual dispatch only while the conformance test suite is under rapid development
+  push:
+    branches:
+      - develop
+  pull_request:
   workflow_dispatch:
 
 name: Conformance test