APply a nix override

pkgs/default.nix

@@ -1,9 +1,15 @@
-{ callPackage }:
+{ callPackage, nixUnstable }:
 {
   # Create a cpio archive comprised of many initrd's appended to
   # each other. Harder to debug, and requires recursive Nix support,
   # but faster for iterating.
-  makeCpioRecursive = callPackage ./cpio-recursive {};
+  makeCpioRecursive = callPackage ./cpio-recursive {
+    # nixUnstable may not be required. Todo: revisit (2020-05-25)
+    nix = nixUnstable;
+  };
 
-  makeSquashfsManifest = callPackage ./squashfs-recursive {};
+  makeSquashfsManifest = callPackage ./squashfs-recursive {
+    # nixUnstable may not be required. Todo: revisit (2020-05-25)
+    nix = nixUnstable;
+  };
 }

pkgs/squashfs-recursive/default.nix

@@ -1,10 +1,10 @@
-{ runCommand, nixUnstable, jq, path }:
+{ runCommand, nix, jq, path }:
 let
   map-squash = ./map-squash.nix;
 
   mkSquashfsManifest = { name, storeContents, reverse ? false }:
     runCommand "${name}-squashfs-manifest" {
-      buildInputs = [ nixUnstable jq ];
+      buildInputs = [ nix jq ];
       requiredSystemFeatures = [ "recursive-nix" ];
       exportReferencesGraph = [ "root" storeContents ];
       NIX_PATH = "nixpkgs=${path}";