(virtualbox) Always install $ENV:PROGRAMFILES to the PATH

Looks like VirtualBox installs to $ENV:PROGRAMFILES on 32- and 64-bit systems now. So, this change _should_ work, but I didn't test it end-to-end. I know I should have kept this commit smaller, but I had to break out some variables make sense of everything ;)
Yikes... Maybe we need to have this discussion in an Issue? Aren't we _always_ running in an elevated prompt now? Or is that just the recommendation?