Fix trust policy

Author: sarahchen6

.github/chainguard/self.update-system-tests.push.sts.yaml

@@ -1,6 +1,6 @@
 issuer: https://token.actions.githubusercontent.com
 
-subject: repo:DataDog/dd-trace-java:ref:refs/(heads/master|tags/v[0-9]+.[0-9]+.0)
+subject_pattern: repo:DataDog/dd-trace-java:ref:refs/(heads/master|tags/v[0-9]+.[0-9]+.0)
 
 claim_pattern:
   event_name: (push|workflow_dispatch)