ci: windows: Copy the styles directory when packaging

Qt can make use of qwindowsvistastyle.dll if present, and our MinGW
container has the library, but it was not being copied during the
packaging process. Thus, yuzu looked like a Windows 98 application when
using the PR-verify artifacts.

This copies over the DLL during packaging, for that sweet-sweet Windows
Vista style.

In addition, set the Qt plugins path instead of the plugins/platforms
path. This way we can use the directory directly, rather than appending
a `..` everytime we need something just outside of it.