Add a nix-flake-repl script.

nix/configuration/roles/nix_repl/default.nix

@@ -0,0 +1,38 @@
+{
+  config,
+  lib,
+  pkgs,
+  self,
+  ...
+}:
+
+let
+  nix-flake-repl = pkgs.writeShellScriptBin "nix-flake-repl" ''
+    source /etc/set-environment
+    target="''${1:-$(pwd)}"
+    nix repl --expr "builtins.getFlake \"''$target\""
+  '';
+  nix-self-repl = pkgs.writeShellScriptBin "nix-self-repl" ''
+    source /etc/set-environment
+    nix repl --expr 'builtins.getFlake "${self}"'
+  '';
+in
+{
+  imports = [ ];
+
+  options.me = {
+    nix_repl.enable = lib.mkOption {
+      type = lib.types.bool;
+      default = false;
+      example = true;
+      description = "Whether we want to install nix_repl.";
+    };
+  };
+
+  config = lib.mkIf config.me.nix_repl.enable {
+    environment.systemPackages = [
+      nix-flake-repl
+      nix-self-repl
+    ];
+  };
+}