Fix broken tmux default shell

I don't understand the issue, I thought tmux would inspect env $SHELL.
Looks like it doesn't and use sh instead.
So we configure to use zsh instead.

home.nix

@@ -80,6 +80,7 @@
       inherit config;
     };
     tmux = import ./home/programs/tmux.nix {
+      inherit pkgs;
     };
     zsh = import ./home/programs/zsh.nix {
       inherit config;

home/programs/tmux.nix

@@ -1,4 +1,4 @@
-{ }:
+{ pkgs }:
 
 {
   enable = true;
@@ -16,6 +16,8 @@
   # Also see https://github.com/neovim/neovim/wiki/FAQ#colors-arent-displayed-correctly
   terminal = "tmux-256color";
 
+  shell = "${pkgs.zsh}/bin/zsh";
+
   sensibleOnTop = false;
 
   extraConfig = builtins.readFile ../files/tmux;