scripts: ci: check_compliance: ruff: avoid mixing stdout and stderr

The python subprocess call had a stdout=subprocess.PIPE parameter that
redirects standard output to a pipe, and a stderr=subprocess.STDOUT
parameter that redirected stderr to the pipe. This mixed JSON and
non-JSON output together, and issued an exception. Fixing with
stderr=subprocess.DEVNULL to ignore standard error and only keep the
JSON output.

Signed-off-by: Josuah Demangeon <me@josuah.net>