nix: move overlay to the top level

flake.nix

@@ -6,16 +6,13 @@
 
   outputs = { self, nixpkgs, utils }:
     let
-      cargoToml = with builtins; (fromTOML (readFile ./Cargo.toml));
+      inherit (builtins) fromTOML readFile substring;
 
-      version = "${cargoToml.package.version}_${builtins.substring 0 8 self.lastModifiedDate}_${self.shortRev or "dirty"}";
+      cargoToml = fromTOML (readFile ./Cargo.toml);
-    in
+      version = "${cargoToml.package.version}_${substring 0 8 self.lastModifiedDate}_${self.shortRev or "dirty"}";
-    utils.lib.eachDefaultSystem (system:
-      let
-        inherit (nixpkgs) lib;
-        pkgs = import nixpkgs { inherit system; };
 
-        vnetod = pkgs.rustPlatform.buildRustPackage {
+      mkVnetod = { lib, rustPlatform, ... }:
+        rustPlatform.buildRustPackage {
           name = "vnetod-${version}";
 
           src = lib.cleanSource ./.;
@@ -23,6 +20,17 @@
 
           doCheck = true;
         };
+    in
+    {
+      overlay = final: prev: {
+        vnetod = final.callPackage mkVnetod { };
+      };
+    }
+    // utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = import nixpkgs { inherit system; };
+
+        vnetod = pkgs.callPackage mkVnetod { };
 
         docker = pkgs.dockerTools.buildLayeredImage {
           name = "pleshevskiy/vnetod";
@@ -41,7 +49,7 @@
         };
 
         packages = {
-          inherit docker;
+          inherit docker vnetod;
           default = vnetod;
         };
 
@@ -49,7 +57,5 @@
           packages = with pkgs; [ cargo rustc rustfmt clippy rust-analyzer ];
           RUST_SRC_PATH = pkgs.rustPlatform.rustLibSrc;
         };
-
-        overlays = f: p: { inherit vnetod; };
       });
 }