appveyor.yml: try updating msys2 to fix failing mingw-w64-x86_64 build

https://github.com/harfbuzz/harfbuzz/pull/1093#issuecomment-405201903
https://ci.appveyor.com/project/harfbuzz/harfbuzz/build/job/ky7lao1ii1bi7ew4#L71
1 file changed