Allow changing `update_method` to rsync via CLI

[Ma27]

git-bundle is the better way to deploy things given its correctness guarantees, however it's not practical for dev cycles.

So what I usually do is to edit the environment.cfg accordingly. Now, this is a little tedious, gets forgotten on commits which is why I'm wondering if it'd make sense to allow overriding this value with a flag from CLI.

[elikoga]

Maybe something general purpose for editing environment config even