Apply mergability check should not fail if the "digger/apply" status check is the only thing blocking

[motatoes]

In #1150 we have implemented an aggregate status check (#959) for digger digger/plan and digger/apply. Although those are set correctly there is an additional mergability check which causes digger apply step to fail IF the PR is not mergable.

This has introduced chicken-egg situation where you want digger/apply check to succeed for PR to be mergable but digger/apply needs to complete

We should do an additional check on mergability where if PR state is "blocked" and the only check blocking it is "digger/apply" then to still proceed with the apply

[liamwitt]

What's the current workaround for this in terms of ensuring that digger apply has run before a PR is merged? It seems as though it is feasible for changes to not be applied before merge with human error.

[motatoes]

No workaround yet unfortunately until we ship a check to ignore that status check