Use explicit nixpkgs

Author: igor-ramazanov

flake.lock

@@ -8,6 +8,10 @@
       };
     };
 
+    nixpkgs = {
+      url = "github:NixOS/nixpkgs";
+    };
+
     flake-utils = {
       url = "github:numtide/flake-utils";
       inputs.systems.follows = "systems";