Fixes case where  `shopify.downloadTheme` is broken on first try

`shopify.downloadTheme` was calling `shopify.download` with original Shopify key. Passing this through `shopify._makeAssetKey` resulted in a relative URL, e.g. "../assets/logo.png", which is an invalid key.

By joining this key with base path we retain original interface of `shopify.download`, and in turn the `shopify:download` task.