Run sudo with -s in the darwin module (#807)

Currently activation is run with `sudo -i` this defaults to the user's
login shell. This can lead to problems if the user's shell isn't set
properly.

By passing `-s` rather than `-i`, `sudo` runs `activate` in `SHELL`
instead. We assume that at this point in the activation `SHELL`
contains the path to a bash in the nix store. This should always be a
valid shell to run the `activate` script with.

From the `sudo` manual it seems like this cannot be fixed if `SHELL`
isn't set at this point or by passing a command to `-s` because that
command is then passed to the user's shell.

nix-darwin/default.nix

@@ -134,7 +134,7 @@ in
     system.activationScripts.postActivation.text =
       concatStringsSep "\n" (mapAttrsToList (username: usercfg: ''
         echo Activating home-manager configuration for ${username}
-        sudo -u ${username} -i ${pkgs.writeShellScript "activation-${username}" ''
+        sudo -u ${username} -s ${pkgs.writeShellScript "activation-${username}" ''
           ${lib.optionalString (cfg.backupFileExtension != null)
             "export HOME_MANAGER_BACKUP_EXT=${lib.escapeShellArg cfg.backupFileExtension}"}
           ${lib.optionalString cfg.verbose "export VERBOSE=1"}