Make MSVC and MinGW MSI installers use separate prefixes
Currently they both install into
x86, which means they clobber each other. This can happen because of any of these reasons:
People don't read the download page and just install everything they can see on that page
People install the MinGW binaries then after a few months they've forgotten about it and they install the MSVC binaries
People sometimes legitimately want to install and use both
If we install both into separate prefixes, this will resolve everything.