Prevent running bazel-diff from updating Bazel lockfile

Tested at https://buildkite.com/bazel/google-bazel-presubmit/builds/76429#018d4011-f8f5-421c-9974-235a65623af8
diff --git a/buildkite/bazelci.py b/buildkite/bazelci.py
index 9c08a3d..5c27d08 100755
--- a/buildkite/bazelci.py
+++ b/buildkite/bazelci.py
@@ -2469,6 +2469,7 @@
                     repo_dir,
                     "-b",
                     bazel_binary,
+                    "--bazelCommandOptions=--lockfile_mode=off",
                     json_path,
                 ],
                 capture_stderr=True,