Fix RELEASE_BRANCH_RE (#2721)

buildkite/bazelci.py

diff --git a/buildkite/bazelci.py b/buildkite/bazelci.py
index e927687..6f791f1 100755
--- a/buildkite/bazelci.py
+++ b/buildkite/bazelci.py
@@ -560,7 +560,7 @@
 )
 
 COMMIT_RE = re.compile(r"[0-9a-z]{40}")
-RELEASE_BRANCH_RE = re.compile(r"\brelease-(\d+\.\d+\.\d+(rc\d+)?)\b")
+RELEASE_BRANCH_RE = re.compile(r"\b(release-\d+\.\d+\.\d+(rc\d+)?)\b")
 
 CONFIG_FILE_EXTENSIONS = {".yml", ".yaml"}