CI: Add MinGW builds

[bjorn]

This won't work yet, at the least because we still need to figure out how to make CMake use the installed MinGW.

This also fixes the Windows 32-bit build to actually build against a 32-bit Qt.

[bjorn]

Ah, it was going so well! Until...

[ 25%] Building CXX object aseprite/third_party/cityhash/CMakeFiles/cityhash.dir/src/city.cc.obj
D:\a\qaseprite\qaseprite\aseprite\third_party\cityhash\src\city.cc:92:10: fatal error: byteswap.h: No such file or directory
   92 | #include <byteswap.h>
      |          ^~~~~~~~~~~~
compilation terminated.

And another, which should be easy to fix:

[ 18%] Building CXX object aseprite/laf/base/CMakeFiles/laf-base.dir/win/ver_query_values.cpp.obj
D:\a\qaseprite\qaseprite\aseprite\laf\base\win\ver_query_values.cpp: In function 'std::map<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char> > base::ver_query_values(const wchar_t*)':
D:\a\qaseprite\qaseprite\aseprite\laf\base\win\ver_query_values.cpp:48:15: error: 'uint8_t' was not declared in this scope
   48 |   std::vector<uint8_t> data(size);
      |               ^~~~~~~
D:\a\qaseprite\qaseprite\aseprite\laf\base\win\ver_query_values.cpp:20:1: note: 'uint8_t' is defined in header '<cstdint>'; did you forget to '#include <cstdint>'?
   19 | #include <windows.h>
  +++ |+#include <cstdint>
   20 |