Use default Bazel binary in incompatible flags pipeline (#520)

* Use default Bazel binary in incompatible flags pipeline

Change-Id: I341057e95fe95fb8b289b3e45b5ebd4130a250ca

* Address comment

Change-Id: I7be6c5f7ef51610daf226847ac9a593b40508d77
diff --git a/buildkite/bazelci.py b/buildkite/bazelci.py
index 16a35a6..12e37c0 100644
--- a/buildkite/bazelci.py
+++ b/buildkite/bazelci.py
@@ -556,6 +556,11 @@
     incompatible_flags,
     bazel_version=None,
 ):
+    # If we want to test incompatible flags, we ignore bazel_version and always use
+    # the latest Bazel version through Bazelisk.
+    if incompatible_flags:
+        bazel_version = None
+
     # TODO(https://github.com/bazelbuild/bazel/issues/7555): remove this hack once the
     # latest Bazel release is no longer broken.
     bazel_version = bazel_version or "0.22.0"