overlay: make finalNix depend on the nix version being used by a package

This should address both of the following (were 35 is more general and
36 a specific case)

- lix-project/nixos-module#35
- lix-project/nixos-module#36

overlay.nix

@@ -62,8 +62,22 @@ let
 
   overlay =
     lib.genAttrs overridelist_upstream (
-      name: if (lib.functionArgs prev.${name}.override ? "nix") then prev.${name}.override {
-        nix = final.nixVersions.stable_upstream;
+      name:
+      if (lib.functionArgs prev.${name}.override ? "nix") then
+        let
+          # Get the two common inputs of a derivation/package.
+          inputs = prev.${name}.buildInputs ++ prev.${name}.nativeBuildInputs;
+          nixDependency = lib.findFirst
+            (drv: lib.hasPrefix "nix-2." drv.name)
+            final.nixVersions.stable # default to stable if nix is not an input
+            inputs;
+          nixMajor = lib.versions.major nixDependency;
+          nixMinor = lib.versions.minor nixDependency;
+          nixAttr = "nix_${nixMajor}_${nixMinor}";
+          finalNix = final.nixVersions.${nixAttr};
+        in
+        prev.${name}.override {
+          nix = finalNix;
         } else prev.${name}
     ) // {
     lix-overlay-present = 1;