move path filtering to a CI job instead

[alaviss]

Github's brilliant path filtering system is incompatible with
it's required checks system, so as a workaround we move to an userspace
version which won't have this issue.

A new 'All check passes' job is added to match against in required
checks since the feature doesn't work if the matrix isn't evaluated (ie.
when its skipped).

Tested with a mock PR which worked: alaviss#5

Will require adjustment of the required checks constraints to All check passes since the matrix won't evaluate if the job is skipped and Github won't see the required macos-latest check...