Enable new parallel_emerge impl for PCQ

Enable the new thin wrapper implementation of parallel_emerge on the PCQ
only to test the stability of the new implementation.

BUG=chromium:989962
TEST=None

Change-Id: I3ae662a7d1f9e2eeb34ae4b2e89ba8b823bd7113
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/1739164
Tested-by: Chris McDonald <cjmcdonald@chromium.org>
Commit-Queue: Chris McDonald <cjmcdonald@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Alex Klein <saklein@chromium.org>
diff --git a/service/sysroot.py b/service/sysroot.py
index 106641f..17d4452 100644
--- a/service/sysroot.py
+++ b/service/sysroot.py
@@ -296,6 +296,7 @@
   with osutils.TempDir(base_dir='/tmp') as tempdir:
     extra_env = {
         constants.CROS_METRICS_DIR_ENVVAR: tempdir,
+        'USE_NEW_PARALLEL_EMERGE': '1',
     }
 
     if run_configs.use_flags: