Fix low_bandwidth_audio_test on Windows bots.

Bug: b/197492097
Change-Id: I29ae4d699c3c928f9d22da8cf9026c462f3afa7e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/239540
Reviewed-by: Christoffer Jansson <jansson@google.com>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Jeremy Leconte <jleconte@google.com>
Cr-Commit-Position: refs/heads/main@{#35452}

tools_webrtc/mb/mb_unittest.py

@@ -458,6 +458,7 @@ class UnitTest(unittest.TestCase):
         'base_unittests', 'base_unittests_script.py',
     ])
     self.assertEqual(command, [
+        'vpython',
         '../../base/base_unittests_script.py',
     ])