The `specialArgs` defined in `hosts` has kept some modules from working
in external flakes. Instead, we simply enumerate packages in `hosts`
which should be pulled from `unstablePkgs`.
The current qutebrowser configuration is sufficiently complex that it
benefits from being factored out into a separate subprofile. An
additional benefit is that this subprofile is now exported via the
`nixosModules` flake output.
