Switch to using SSH urls

Haven't been able to test because of 404s on the mock-res dependency, so
this probably doesn't work in its current state
I've just realised a problem: if `RECEIVER_USE_SSH` is false, `repository.url` (the https one) will be used, and it won't be checked. Maybe this function should check the `ssh_url` by default, or the `url` if `RECEIVER_USE_SSH` is false?