Begin reporting Test Discovery Agent metrics

Start running the Test Discovery Agent and reportings its results via
metrics. No changes to the actual build process yet.

Test: atest build_test_suites_test && atest optimized_targets_test
Bug: 372973116
Change-Id: I958e034985c41ede8d6a2700311a4dd6e7ac18ba
diff --git a/ci/test_discovery_agent.py b/ci/test_discovery_agent.py
index 4eed28d..008ee47 100644
--- a/ci/test_discovery_agent.py
+++ b/ci/test_discovery_agent.py
@@ -17,7 +17,6 @@
 import logging
 import os
 import subprocess
-import buildbot
 
 
 class TestDiscoveryAgent:
@@ -50,7 +49,7 @@
       A list of test zip regexes that TF is going to try to pull files from.
     """
     test_discovery_output_file_name = os.path.join(
-        buildbot.OutDir(), self._TRADEFED_DISCOVERY_OUTPUT_FILE_NAME
+        os.environ.get('TOP'), 'out', self._TRADEFED_DISCOVERY_OUTPUT_FILE_NAME
     )
     with open(
         test_discovery_output_file_name, mode="w+t"