[Experimental] Use cquery before bazel-diff (#1626)

Progress towards #1605
diff --git a/buildkite/bazelci.py b/buildkite/bazelci.py
index c6925ac..ff40285 100755
--- a/buildkite/bazelci.py
+++ b/buildkite/bazelci.py
@@ -2154,7 +2154,7 @@
         + [
             "--nosystem_rc",
             "--nohome_rc",
-            "query",
+            "cquery" if os.getenv("EXP_USE_CQUERY") else "query",
             get_test_query(test_targets, test_flags),
         ],
         print_output=False,