ci: build Neovim with MSVC on Windows, matching upstream

[justinmk]

Problem:

Windows test-upstream build fails:

…/libuv/src/uv-common.c:960:26: warning: passing argument 1 of 'uv__free' discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
  960 |     uv__free(cpu_infos[i].model);
      |              ~~~~~~~~~~~~^~~~~~
…/libuv/src/uv-common.c:81:21: note: expected 'void *' but argument is of type 'const char *'
   81 | void uv__free(void* ptr) {
      |               ~~~~~~^~~

The test-upstream Windows job builds Neovim with MinGW/GCC via msys2, but Neovim's own CI uses MSVC.

Solution:

Use MSVC/Ninja to build Neovim (matching upstream test_windows.yml), and only use msys2 for running oldtest which requires make.