nlohmann_json: allow overrides by eliminating use of rec

The existing nlohmann_json expression prevents users from overriding
fields (like `doCheck`) because it uses `rec` in the argument to
`mkDerivation`.  Let's use the shiny new
pass-a-function-to-`mkDerivation` feature to do the same thing without
blocking overrides.

pkgs/development/libraries/nlohmann_json/default.nix

@@ -10,23 +10,23 @@ let
     rev = "v3.0.0";
     sha256 = "O6p2PFB7c2KE9VqWvmTaFywbW1hSzAP5V42EuemX+ls=";
   };
-in stdenv.mkDerivation rec {
+in stdenv.mkDerivation (finalAttrs: {
   pname = "nlohmann_json";
   version = "3.10.5";
 
   src = fetchFromGitHub {
     owner = "nlohmann";
     repo = "json";
-    rev = "v${version}";
+    rev = "v${finalAttrs.version}";
     sha256 = "DTsZrdB9GcaNkx7ZKxcgCA3A9ShM5icSF0xyGguJNbk=";
   };
 
   nativeBuildInputs = [ cmake ];
 
   cmakeFlags = [
-    "-DBuildTests=${if doCheck then "ON" else "OFF"}"
+    "-DBuildTests=${if finalAttrs.doCheck then "ON" else "OFF"}"
     "-DJSON_MultipleHeaders=ON"
-  ] ++ lib.optional doCheck "-DJSON_TestDataDirectory=${testData}";
+  ] ++ lib.optional finalAttrs.doCheck "-DJSON_TestDataDirectory=${testData}";
 
   doCheck = stdenv.hostPlatform == stdenv.buildPlatform;
 
@@ -44,4 +44,4 @@ in stdenv.mkDerivation rec {
     license = licenses.mit;
     platforms = platforms.all;
   };
-}
+})