Add a command to launch a repl of the current flake.

nix/configuration/flake.nix

@@ -90,6 +90,30 @@
                 ];
               };
             }
+            (
+              {
+                config,
+                lib,
+                pkgs,
+                ...
+              }:
+              let
+                repl_path = toString ./.;
+                nix-self-repl = pkgs.writeShellScriptBin "nix-self-repl" ''
+                  source /etc/set-environment
+                  nix repl "${repl_path}/repl.nix" "$@"
+                '';
+                # If we wanted the current version of a flake then we'd just launch
+                #   nix repl
+                # and then run:
+                #   :lf /path/to/flake
+              in
+              {
+                config = {
+                  environment.systemPackages = lib.mkIf config.nix.enable [ nix-self-repl ];
+                };
+              }
+            )
           ];
         }
       ) nodes;