Add a shared nixos module for configuring whitby as a binary nix cache,
and refactor tverskoy to use this module.
This is enabled via an option to pave the way for including it as an
import in all depot-generated nixos configs at some point in the future.
Change-Id: I6dcc0e8eb48b1ac34457666dceebeedd5da6c526
Reviewed-on: https://cl.tvl.fyi/c/depot/+/4344
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>
Reviewed-by: wpcarro <wpcarro@gmail.com>
Autosubmit: grfn <grfn@gws.fyi>
