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.