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>

scripts/ci/check_compliance.py

@@ -1657,7 +1657,7 @@ class Ruff(ComplianceTest):
                     f"ruff check --force-exclude --output-format=json {file}",
                     check=True,
                     stdout=subprocess.PIPE,
-                    stderr=subprocess.STDOUT,
+                    stderr=subprocess.DEVNULL,
                     shell=True,
                     cwd=GIT_TOP,
                 )