[CI] Mark MinGW build failures allowed

Currently we are requiring MinGW tests to pass on AppVeyor. Almost nobody will use MinGW builds if regular MSVC builds work fine. So it should be on the onus of the few people caring about MinGW to ensure that the build chain works on those platforms.

Example here, apparently the uriparser library doesn't build on MinGW:
https://ci.appveyor.com/project/pitrou/arrow/build/job/t64xwyj2axhl1jgr

There is a tendency to inflate the number of different configurations in our CI matrices. Not only it makes builds longer and adds delays (see how long you have to wait before you get a CI result on AppVeyor), but it's of dubious utility. I'm not sure it serves the project's general interest.

Rant off