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

appveyor.yml

@@ -29,6 +29,8 @@ environment:
       MSYS2_ARCH: i686
 
 install:
+  - 'if "%compiler%"=="msys2" C:\msys64\usr\bin\bash -lc "pacman -Syu --noconfirm"'
+  - 'if "%compiler%"=="msys2" C:\msys64\usr\bin\bash -lc "pacman -Su --noconfirm"'
   - C:\msys64\usr\bin\bash -lc "pacman --noconfirm -S mingw-w64-x86_64-ragel"
 
 build_script: