fix(sway): add possibility to not install swaylock package

Useful when using home-manager on third-party distribution. Swaylock
from nixpkgs become unable to unlock session because PAM
incompatibilities.

modules/home-manager/desktop/sway/includes/sway.nix

@@ -76,7 +76,7 @@ in
           "${mod}+Alt+k" = "kill";
           "${mod}+Alt+c" = "reload";
           "${mod}+Alt+Backspace" = "exec swaynag -t warning -m 'Exit sway?' -B 'Yes' 'swaymsg exit'";
-          "${mod}+Alt+l" = "exec ${pkgs.swaylock}/bin/swaylock";
+          "${mod}+Alt+l" = if cfg.swaylock.useNullPackage then "exec swaylock" else "exec ${pkgs.swaylock}/bin/swaylock";
         # Manage windows placement
           "${mod}+b" = "splith";
           "${mod}+v" = "splitv";