fix formatting_check CI config

[wisp3rwind]

cf. #4654, the formatting_check.yml is invalid and never runs, immediately aborting on parsing this configuration file.

[wisp3rwind]

@Serene-Arc do we actually need the pox GH action? In our main CI config, as far as I can tell, we just directly execute tox. pox also seems to be essentially unmaintained (although that might not be a problem in principle; maybe this is really just done and doesn't require much maintenance). However, it does trigger some deprecation warnings, cf. https://github.com/beetbox/beets/actions/runs/6602808494

[Serene-Arc]

@wisp3rwind probably not? I didn't realise the action was broken, sorry for that. I tried to keep it as close to the existing one as possible. It's likely that no, we don't need it.

[wisp3rwind]

@wisp3rwind probably not? I didn't realise the action was broken, sorry for that. I tried to keep it as close to the existing one as possible. It's likely that no, we don't need it.

No problem; I was just a little confused about the CI failures I got for #4962 :)