Detect shifter lint errors in prechecker/postchecker

Following from MDLSITE-4323 I made it possible to detect shifter lint issues:

https://github.com/danpoltawski/moodle-local_ci/compare/shifter-prechecker

A note about the current implementation:

I could be clever about getting the full shifter verbose output and parsing to work exact lines affected by a failing lint check. That would mean we could apply diff'ing solution and avoid reporting unrelated errors to a patch. But I am not, because:
a) I think it would be fragile
b) I think it is reasonable that we make sure shifter never reports errors when built. The ugly 'err' reported in output should be exceptional.

Unfortunately this is blocked by MDL-45364 so wasn't merged. We should also do it on the post checker.