Increase timeout for successful signing

[diox]

In https://github.com/mozilla/addons-server/issues/12622 AMO is going to refactor the way unlisted submissions are handled, and have them go through the same auto-approval process as listed ones.

Validation and signing will continue to be automatic, but signing will no longer directly follow validation and may take more time to complete than before. This is an issue because sign-addon expects signing to be almost instantaneous and has a 2 minutes timeout before reporting an error. That timeout needs to be updated to a higher value to leave time for signing to succeed.

The value of the timeout we need is currently being discussed, I'll add it to this issue as soon as possible.