Fix dispatcher test policy to ensure we have the proper subject (#22033)

Author: AAraKKe

.github/chainguard/self.test-dispatcher.sts.yaml

@@ -1,6 +1,6 @@
 issuer: https://token.actions.githubusercontent.com
 
-subject: repo:DataDog/integrations-core:ref:refs/heads/master
+subject_pattern: repo:DataDog/integrations-core:(pull_request|ref:refs/heads/master)
 
 claim_pattern:
   job_workflow_ref: DataDog/integrations-core/.github/workflows/trigger-pr-checks.yaml