src ext copy publishes extension with sourcegraph.com url

Created by: tjkandala

See sourcegraph/customer#784. src ext copy does not work for air-gapped instances since we do not remove the url property from the manifest, which points to the extension bundle on sourcegraph.com instead of the private registry's extension bundle (which is used when url is omitted). This makes it so src ext copy basically just pins a Cloud extension's version in a private registry instead of truly preventing requests to sourcegraph.com.