bisector: Add tag to bisector-created builds

Bug: b/401921575
Change-Id: I0b0c37cc511f452322ca51791c3f88a36ac40a33
Reviewed-on: https://pigweed-review.googlesource.com/c/infra/recipes/+/275213
Reviewed-by: Ted Pudlik <tpudlik@google.com>
Pigweed-Auto-Submit: Rob Mohr <mohrr@google.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/recipes/bisector.py b/recipes/bisector.py
index 77174f6..1778b1e 100644
--- a/recipes/bisector.py
+++ b/recipes/bisector.py
@@ -324,6 +324,9 @@
                     id=trigger.commit,
                     ref=f'refs/heads/{trigger.ref}',
                 ),
+                tags=[
+                    common_pb.StringPair(key='user_agent', value='bisector'),
+                ],
             ),
         )