terraria service: fix tmux output

tmux.bin was removed in 5535d9439415454abe00589bd7575b61e7fbb8db

Use `lib.getBin` to be more robust to future changes.

nixos/modules/services/games/terraria.nix

···
         User    = "terraria";
         Type = "oneshot";
         RemainAfterExit = true;
-        ExecStart = "${pkgs.tmux.bin}/bin/tmux -S /var/lib/terraria/terraria.sock new -d ${pkgs.terraria-server}/bin/TerrariaServer ${concatStringsSep " " flags}";
-        ExecStop = "${pkgs.tmux.bin}/bin/tmux -S /var/lib/terraria/terraria.sock send-keys Enter \"exit\" Enter";
+        ExecStart = "${getBin pkgs.tmux}/bin/tmux -S /var/lib/terraria/terraria.sock new -d ${pkgs.terraria-server}/bin/TerrariaServer ${concatStringsSep " " flags}";
+        ExecStop = "${getBin pkgs.tmux}/bin/tmux -S /var/lib/terraria/terraria.sock send-keys Enter \"exit\" Enter";
       };
 
       postStart = ''