Add rubocop linter for Ruby (#21934)

Use gemdir and pname as suggested by @zimbatm

Fix silly typo

pkgs/development/tools/rubocop/default.nix

@@ -0,0 +1,17 @@
+{ stdenv, lib, bundlerEnv, ruby, makeWrapper }:
+
+bundlerEnv rec {
+  pname = "rubocop";
+
+  inherit ruby;
+
+  gemdir = ./.;
+
+  meta = with lib; {
+    description = "Automatic Ruby code style checking tool";
+    homepage = http://rubocop.readthedocs.io/en/latest/;
+    license = licenses.mit;
+    maintainers = with maintainers; [ leemachin ];
+    platforms = platforms.unix;
+  };
+}