Or even better, moved into release-common.nix which exists exactly for this purpose.

@edolstra But release-common.nix takes a pkgs argument itself and is called with different pkgs arguments in release.nix. Something like this would be possible for release-common.nix:

{ nixpkgs     ? null
, nixpkgsArgs ? {}
}:

with if nixpkgs != null
  then import nixpkgs nixpkgsArgs
  else import (builtins.fetchGit ...) nixpkgsArgs;

rec {
  inherit pkgs,
  ...
}

Then release.nix could look like this

{ nixpkgs ? null
, ...
}:
...
  build = pkgs.lib.genAttrs systems (system:
    with import ./release-common.nix { inherit nixpkgs; nixpkgsArgs = { inherit system; }; };
...

Alternatively release-common could look like this:

{ nixpkgs ? null }: nixpkgsArgs:
...

and release.nix like this:

{ nixpkgs ? null
, ...
}:
releaseCommon = import ./release-common.nix {inherit nixpkgs;}
...
  build = pkgs.lib.genAttrs systems (system:
    with releaseCommon { inherit system; };

I am very new to nix and do not know which would be the most idiomatic. To me a separate version.nix or nixpkgs.nix seemed the most straight-forward.

This has been resolved by moving to flakes and flakes-compat.