maybe release.nix is fixed now?

.nix/derivation.nix

@@ -7,7 +7,7 @@
 , clang_12
 , clang-tools
 , gnutls
-, keldu
+, forstio
 }:
 
 stdenvNoCC.mkDerivation {
@@ -21,7 +21,7 @@ stdenvNoCC.mkDerivation {
   ];
 
   buildInputs = [
-    keldu.forstio
+    forstio
   ];
 
   src = ./..;

.nix/release.nix

@@ -1,12 +1,18 @@
-{ nixpkgs ? import <nixpkgs>
+{ nixpkgs ? <nixpkgs>
 , forstio
 , ...
 }:
 let
-  pkgs = nixpkgs;
+  pkgs = import nixpkgs {};
+
+  forstioJobset = import "$(forstio)"/.nix/release.nix {
+    inherit nixpkgs;
+  };
 in
 {
   keldu = {
-    forstio-window = pkgs.callPackage ./derivation.nix {};
+    forstio-window = pkgs.callPackage ./derivation.nix {
+      forstio = forstioJobset.keldu.forstio;
+    };
   };
 }