Try enabling `unsafeDiscardReferences` for building `disk` after upgrade to nixpkgs 25.xx

[yfyf]

Can reduce build time by not scanning for runtime dependencies (the built disk image has none): https://nix.dev/manual/nix/2.18/language/advanced-attributes#adv-attr-unsafeDiscardReferences

unsafeDiscardReferences requires structuredAttrs, which did not work with runInLinuxVM (which is how the disk image is built), but was fixed recently in NixOS/nixpkgs@97ed6b4