Runner should use configured endpoint, not the `external_url`
Zendesk issue: https://gitlab.zendesk.com/agent/tickets/16859 and https://gitlab.zendesk.com/agent/tickets/16646
Runners should use the URL that they were registered with, not the configured external_url or HTTPS clone URL. This is more flexible because runners might be in the same private network, or on an external network. If we support cloning through the URL the runner was configured with, both scenarios are possible. Using the HTTPS clone URL given by GitLab this is not necessarily possible.