fix(nix): make the package code correct

- testDeps should be separate, in nativeBuildInputs to allow for disabling
checks.
- rust should be a native dependency as it doesn't need to be put
nowhere next to the resulting binary.

default.nix

@@ -9,20 +9,6 @@
   darwin,
   python3,
 }: let
-  testDeps = [
-    python3
-  ];
-
-  buildDeps = rust:
-    [
-      rust
-    ]
-    ++ (lib.optionals stdenv.isDarwin [
-      libiconv
-      darwin.apple_sdk.frameworks.Foundation
-    ])
-    ++ testDeps;
-
   mkPackage = rust:
     (makeRustPlatform {
       cargo = rust;
@@ -31,12 +17,22 @@
     .buildRustPackage {
       pname = packageToml.name;
       inherit (packageToml) version;
+
       src = builtins.path {
         path = ./.;
         inherit (packageToml) name;
       };
+
+			dontUseCargoParallelTests = true;
+
       cargoLock.lockFile = ./Cargo.lock;
-      buildInputs = buildDeps rust;
-      dontUseCargoParallelTests = true;
+
+			nativeBuildInputs = [ rust ];
+			buildInputs = (lib.optional stdenv.isDarwin [
+						libiconv
+						darwin.apple_sdk.frameworks.Foundation
+				]);
+
+			nativeCheckInputs = [python3];
     };
 in (mkPackage rust-bin.stable.latest.minimal)