compute: un-hardcode sourcegraphURL endpoint in notebook block

Created by: rvantonder

This will let the compute notebook block talk to the instance's endpoint (sourcegraph.test, sourcegraph.com, customer.instance.sourcegraph.com, etc)

Test plan

Manually tested, since this just propagates existing state (sourcegraphURL) and doesn't add logic. End-to-end test would cover this once there's stable functionality to test.