FindwxWidgets: Restore win32 find style on MinGW

Since commit 6fac8af9ca (FindwxWidgets: set wxWidgets_FIND_STYLE to unix
on MINGW, 2022-04-11, v3.24.0-rc1~296^2), we only tried unix find style
on MinGW.  However, only some MinGW builds have the `wx-config` needed
for unix find style.  Other builds, including those using the official
wxWidgets makefiles, do not provide `wx-config` and need to use win32
find style.  To accommodate both use cases, first use win32 find style
and if it fails, use unix find style.