ci(copybara): set up Zuul to sync with dotfiles-external

GitOrigin-RevId: 5d313e1f2dd57b3c9c8ee317a545559ab9a16c8f
diff --git a/.copybara/test_repos_config.bara.sky.template b/.copybara/test_repos_config.bara.sky.template
new file mode 100644
index 0000000..1281fc2
--- /dev/null
+++ b/.copybara/test_repos_config.bara.sky.template
@@ -0,0 +1,2 @@
+test_origin_url = "file:///path/to/private/dotfiles"
+test_destination_url = "file:///path/to/dotfiles-external"