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

@@ -62,9 +62,23 @@ let
 
   overlay =
     lib.genAttrs overridelist_upstream (
-      name: if (lib.functionArgs prev.${name}.override ? "nix") then prev.${name}.override {
-        nix = final.nixVersions.stable_upstream;
-      } else prev.${name}
+      name:
+      if (lib.functionArgs prev.${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: (drv.pname or "") == "nix")
+            prev.nixVersions.stable_upstream # default to stable nix if nix is not an input
+            inputs;
+          nixMajor = lib.versions.major nixDependency.version;
+          nixMinor = lib.versions.minor nixDependency.version;
+          nixAttr = "nix_${nixMajor}_${nixMinor}";
+          finalNix = final.nixVersions.${nixAttr};
+        in
+        prev.${name}.override {
+          nix = finalNix;
+        } else prev.${name}
     ) // {
     lix-overlay-present = 1;
     # used for things that one wouldn't necessarily want to update, but we