Add `--sudo` flag

[zimbatm]

Is your feature request related to a problem? Please describe.

Deploying system-manager outside is more complicated than needed. This approach also can leave ./result symlinks in the user's working directory that are owned by root. Not a big issue, but also not very elegant.

Describe the solution you'd like

Adopt the same --sudo flag as nixos-rebuild. When --sudo is passed, the program makes a distinction between commands that can run as the user (like the build), and wraps the other ones in sudo (like the activation and nix-env -P)

Additional context

[ntninja]

This has been added recently: 22308ab