Allow disabling the update checker

Is your feature request related to a problem? Please describe. Objection appears to check for updates automatically. This is not helpful to me, as I install software through a package manager on a specific schedule except in emergencies. I prefer not to have any of my software making unnecessary network calls.

Describe the solution you'd like An environment variable, command-line option, or other straightforward way that I can disable update checking when packaging Objection for my system's package manager.

Describe alternatives you've considered I believe the alternative is I need to patch out Python code when packaging.

Additional context See also #744, which makes the problem worse.