fix rebuildstaging edge-case having a read-only URL

This is a somewhat rare case, but it's super easy to automate away, so why not. It's for when you try and push and get

```
fatal: remote error: 
  You can't push to git://github.com/dimagi/django-no-exceptions.git
  Use https://github.com/dimagi/django-no-exceptions.git
```

instead (because you've never pushed to that repo before and git submodule update makes it read-only by default). This will now just silently change the url to be the suggested write-enabled url.