`presubmit.yml`: Explicitly use bazel 6.4.0
diff --git a/.bazelci/presubmit.yml b/.bazelci/presubmit.yml
index df480cc..7f8bdd4 100644
--- a/.bazelci/presubmit.yml
+++ b/.bazelci/presubmit.yml
@@ -1,5 +1,6 @@
 ---
 buildifier: latest
+bazel: 6.4.0
 platforms:
   ubuntu1804:
     environment: