Align flake template with recent API changes and improvements

shell.nix

@@ -1,14 +1,8 @@
 { pkgs ? (
     let
-      inherit (builtins) fromJSON readFile;
-      flakeLock = fromJSON (readFile ./flake.lock);
-      locked = flakeLock.nodes.nixpkgs.locked;
-      nixpkgs = assert locked.type == "github"; builtins.fetchTarball {
-        url = "https://github.com/${locked.owner}/${locked.repo}/archive/${locked.rev}.tar.gz";
-        sha256 = locked.narHash;
-      };
+      inherit (builtins) fetchTree fromJSON readFile;
     in
-    import nixpkgs {
+    import (fetchTree (fromJSON (readFile ./flake.lock)).nodes.nixpkgs.locked) {
       overlays = [
         (import ./overlay.nix)
       ];

templates/app/default.nix

@@ -1,10 +1,20 @@
-{ buildGoApplication, go, lib }:
+{ pkgs ? (
+    let
+      inherit (builtins) fetchTree fromJSON readFile;
+      inherit ((fromJSON (readFile ./flake.lock)).nodes) nixpkgs gomod2nix;
+    in
+    import (fetchTree nixpkgs.locked) {
+      overlays = [
+        (import "${fetchTree gomod2nix.locked}/overlay.nix")
+      ];
+    }
+  )
+}:
 
-buildGoApplication {
-  inherit go;
+pkgs.buildGoApplication {
   pname = "myapp";
   version = "0.1";
+  pwd = ./.;
   src = ./.;
   modules = ./gomod2nix.toml;
-  subPackages = [ "." ];
 }

templates/app/shell.nix

@@ -1,8 +1,22 @@
-{ pkgs ? import <nixpkgs> { } }:
+{ pkgs ? (
+    let
+      inherit (builtins) fetchTree fromJSON readFile;
+      inherit ((fromJSON (readFile ./flake.lock)).nodes) nixpkgs gomod2nix;
+    in
+    import (fetchTree nixpkgs.locked) {
+      overlays = [
+        (import "${fetchTree gomod2nix.locked}/overlay.nix")
+      ];
+    }
+  )
+}:
 
+let
+  goEnv = pkgs.mkGoEnv { pwd = ./.; };
+in
 pkgs.mkShell {
   packages = [
-    pkgs.go
+    goEnv
     pkgs.gomod2nix
   ];
 }