See the discussion starting here:
https://github.com/NixOS/nixpkgs/pull/206951#issuecomment-1364760917
The `darwin.builder` derivation had a gratuitous dependency
on the current Nixpkgs revision due to
`config.system.nixos.revision`. Setting the revision explicitly
to null fixes this problem and prevents the derivation from being
rebuilt on every change to Nixpkgs.
See: https://github.com/NixOS/nixpkgs/issues/108984#issuecomment-1364263324
Before this change the supported platforms were unspecified, so
it would default to being only built on `x86_64-linux`. This
fixes that so that hydra.nixos.org builds and caches the Darwin
build products instead
