ci: Move shellcheck/black/flake8 checks to a new CI job which can fail
We were regressing on Python style too often. Since Python code style is
a lot easier to enforce than C code style, split it (and the shellcheck
checks) out from style-check-diff
into a new CI job which is allowed
to fail the pipeline.
Only trigger it when .sh or .py files have changed, which should reduce resource consumption.
Signed-off-by: Philip Withnall pwithnall@endlessos.org