Add Copybara config for nixpkgs.

third_party/nixpkgs/copy.bara.sky

@@ -0,0 +1,17 @@
+core.workflow(
+    name = "default",
+    origin = git.github_origin(
+        url = "https://github.com/NixOS/nixpkgs.git",
+        ref = "master",
+    ),
+    destination = hg.destination(
+        url = "file:///home/lukegb/depot",
+        fetch = "default",
+        push = "default",
+    ),
+    destination_files = glob(["third_party/nixpkgs/**"], exclude = ["third_party/nixpkgs/copy.bara.sky"]),
+    authoring = authoring.pass_thru("Default email <noreply+nixpkgs@lukegb.com>"),
+    transformations = [
+        core.move("", "third_party/nixpkgs"),
+    ],
+)