vagrant: add darwin support

Vagrant on macOS is distributed as a .dmg installer. Luckily, the
internal contents of that archive resemble that of the .deb we use for
linux. In fact, the similarity is enough that if we move its `embedded`
directory to `opt/vagrant/embedded` and its `bin` to `usr/bin` (and back
again after installation), the derivation's installPhase (which replaces
embedded libs & binaries with those from the package's inputs) can
remain exactly the same between macOS and linux.