Actually we can do it all from a single package.nix

flake.nix

@@ -55,7 +55,7 @@
     });
 
     # A Nixpkgs overlay that provides a 'oott' package.
-    overlays.default = final: prev: {oott = final.callPackage ./nix/package-backend.nix {};};
+    overlays.default = final: prev: {oott = final.callPackage ./nix/package.nix {};};
 
     # Package definition
     packages = forEachSystem (system: {

nix/package-backend.nix

@@ -1,12 +0,0 @@
-{pkgs}:
-pkgs.rustPlatform.buildRustPackage rec {
-  pname = "oott";
-  version = "0.0.1";
-  src = ./../backend;
-  cargoLock = {
-    lockFile = ./../backend/Cargo.lock;
-  };
-
-  nativeBuildInputs = [pkgs.pkg-config];
-  PKG_CONFIG_PATH = "${pkgs.openssl.dev}/lib/pkgconfig";
-}

nix/package.nix

@@ -0,0 +1,20 @@
+{pkgs}: let
+  oott-backend = pkgs.rustPlatform.buildRustPackage rec {
+    pname = "oott-backend";
+    version = "0.0.1";
+    src = ./../backend;
+    cargoLock = {
+      lockFile = ./../backend/Cargo.lock;
+    };
+
+    nativeBuildInputs = [pkgs.pkg-config];
+    PKG_CONFIG_PATH = "${pkgs.openssl.dev}/lib/pkgconfig";
+  };
+in
+  pkgs.stdenv.mkDerivation {
+    name = "oott";
+    buildInputs = [oott-backend];
+    installPhase = ''
+      cp ${oott-backend}/bin/* $out/bin/
+    '';
+  }