Connectivity check should use our URL

(Not 100% sure this is the right repo; we can move it if needed.)

## What Happened

I thought we were already using https://elementary.io/capnet-assist for the connectivity check, but [a user is reporting](https://reddit.com/r/elementaryos/comments/putpup/google_connectivity_check/) it's hitting Google's version (e.g. for Android and Chrome OS) because it's the default in the Ubuntu config.

## Expected Behavior

We should ensure we're hitting our own server here since it's open source and hosted by us instead of Google.