Update NixOS version used for private.storage.

Ideally, we'd automate updating this on a regular cadence. However, we don't currently have confidence in our system tests to do that.

However, until we are doing our pre %Friends & Family testing, we should keep the version up-to-date. That way, we are not too far behind when we first release our software.

The steps here are:

• provide a way to easily update the pinned NixOS version
• update the pinned NixOS version to be current.
• until we are close to launch, update the NixOS version on a weekly cadence.