run_pytest: freeze options

This makes it clear that we don't mutate any of its settings later.

BUG=None
TEST=CQ passes

Change-Id: I29bfedf85bc95705998471e9c8e6c82a57db55d8
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2805491
Reviewed-by: Sergey Frolov <sfrolov@google.com>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/run_pytest.py b/scripts/run_pytest.py
index f7974d2..0420b1a 100644
--- a/scripts/run_pytest.py
+++ b/scripts/run_pytest.py
@@ -23,6 +23,7 @@
 def main(argv):
   parser = get_parser()
   opts = parser.parse_args()
+  opts.Freeze()
 
   pytest_args = opts.pytest_args