Make MSVC and MinGW MSI installers use separate prefixes

Currently they both install into C:\gstreamer\1.0\x86_64 or x86, which means they clobber each other. This can happen because of any of these reasons:

1. People don't read the download page and just install everything they can see on that page

2. People install the MinGW binaries then after a few months they've forgotten about it and they install the MSVC binaries

3. People sometimes legitimately want to install and use both

If we install both into separate prefixes, this will resolve everything.