nix-unit: pin nix version

This will always break and this way we can delay how often we update it.

pkgs/nix-unit/default.nix

@@ -10,6 +10,7 @@
 , cmake
 , clang-tools
 , difftastic
+, makeWrapper
 }:
 
 stdenv.mkDerivation {
@@ -23,7 +24,7 @@ stdenv.mkDerivation {
   };
   buildInputs = [
     nlohmann_json
-    nixVersions.stable
+    nixVersions.nix_2_18
     boost
   ];
   nativeBuildInputs = [
@@ -32,6 +33,7 @@ stdenv.mkDerivation {
     ninja
     # nlohmann_json can be only discovered via cmake files
     cmake
+    makeWrapper
   ] ++ (lib.optional stdenv.cc.isClang [ clang-tools ]);
 
   postInstall = ''