ci: fix chainguard claim regex

Author: hoolioh

.github/chainguard/self.read.members.sts.yaml

@@ -3,7 +3,7 @@ issuer: https://token.actions.githubusercontent.com
 subject_pattern: "repo:DataDog/libdatadog.*"
 
 claim_pattern:
-  ref: "(main|release|julio/.*)" # TODO: remove testing branch and uncomment ref_protected
+  ref: "refs/heads/(main|release|julio/.*)" # TODO: remove testing branch and uncomment ref_protected
   # ref_protected: "true" # TODO: uncomment once tested.
 
 permissions: