fix: write correct install prefix to receipt

[mistydemeo]

This fixes an issue introduced in 91ac6af587cf221b62d56c98cf99fb169c449011, where /bin was always added to this even though the updater expected it to be a root prefix instead. We can then tighten up the special handling around normalizing the root in https://github.com/axodotdev/axoupdater/pull/109, which currently gets confused between whether a directory should end in /bin or if it's a cargo home-style path that shouldn't.