feat(nix): simplify flake.nix, remove devenv

nix/package.nix

@@ -1,8 +1,6 @@
 {
+  target,
   pkgs,
-  nativeRustPlatform,
-  windowsRustPlatform,
-  ...
 }: let
   cargoToml = fromTOML (builtins.readFile ../Cargo.toml);
   name = cargoToml.package.name;
@@ -16,18 +14,14 @@
     meta = {
       inherit (cargoToml.package) description homepage;
     };
+    postBuild = "${pkgs.upx}/bin/upx target/*/release/${name}${target.exeSuffix}";
   };
-  nativeBuild =
-    nativeRustPlatform.buildRustPackage buildArgs
-    // {
-      postBuild = "${pkgs.upx}/bin/upx target/*/release/${name}";
-    };
-  windowsBuild =
-    windowsRustPlatform.buildRustPackage buildArgs
-    // {
-      postBuild = "${pkgs.upx}/bin/upx target/*/release/${name}.exe";
-    };
-in {
-  default = nativeBuild;
-  windows = windowsBuild;
-}
+  rustVersion = pkgs.rust-bin.stable.latest.default.override {
+    targets = [target.triple];
+  };
+  rustPlatform = target.crossPkgs.makeRustPlatform {
+    cargo = rustVersion;
+    rustc = rustVersion;
+  };
+in
+  rustPlatform.buildRustPackage buildArgs