nixpkgs.flake.source

The path to the nixpkgs sources used to build the system. This is automatically set up to be the store path of the nixpkgs flake used to build the system if using nixpkgs.lib.nixosSystem, and is otherwise null by default.

This can also be optionally set if the NixOS system is not built with a flake but still uses pinned sources: set this to the store path for the nixpkgs sources used to build the system, as may be obtained by builtins.fetchTarball, for example.

Note: the name of the store path must be "source" due to https://github.com/NixOS/nix/issues/7075.