Support env. var. that degrades gracefully in case the external world isn't reachable

[marandaneto]

Context: https://github.com/getsentry/team-mobile/issues/3 DACI: https://www.notion.so/sentry/Flag-to-disable-asset-uploads-and-release-creation-5aa83935006a4986a290dd1aa442bedb

There are no implementation details or specs, it's just important that one is able to export an env. var. and be able to have a successful response even if sentry.io isn't reachable.

• [ ] Implement env. var. that degrades gracefully.
• [ ] Log out if the env. var. is active, so they don't forget to unset once everything is back.
• [ ] Log out if the env. var. isn't exported and let the users know that they can export that env. var. in case of errors.
• [ ] Add to the docs https://docs.sentry.io/product/cli/configuration/