feat: switch to fish as an interactive shell

Now that it is written in Rust I am interested!

modules/nixos/default.nix

@@ -35,6 +35,17 @@ in
     ];
 
     programs = {
+      # Launch fish if shell is interactive (from https://wiki.nixos.org/wiki/Fish)
+      bash.interactiveShellInit = # bash
+        ''
+          if [[ $(${pkgs.procps}/bin/ps --no-header --pid=$PPID --format=comm) != "fish" && -z ''${BASH_EXECUTION_STRING} ]]
+          then
+            shopt -q login_shell && LOGIN_OPTION='--login' || LOGIN_OPTION=""
+            exec ${pkgs.fish}/bin/fish $LOGIN_OPTION
+          fi
+        '';
+      # Default shell
+      fish.enable = true;
       # Shell prompt
       starship = {
         enable = true;
@@ -58,8 +69,6 @@ in
           (import ./starship-shorter-text.nix)
         ];
       };
-      # Default shell
-      zsh.enable = true;
     };
 
     environment.etc = keysFromGithub;
@@ -74,7 +83,6 @@ in
         builtins.attrNames keysFromGithub
       );
     };
-    users.defaultUserShell = pkgs.zsh;
     # Open ports for spotifyd
     networking.firewall = {
       allowedUDPPorts = [ 5353 ];