Use the declared original Nixpkgs input in compiler qualification (#931)

source-qualification.nix

@@ -1,8 +1,10 @@
 {
   pkgs,
   library,
+  nixpkgsSource,
   nixpkgsNarHash,
 }:
+assert builtins.isString nixpkgsSource;
 let
   comparisonIdentity = import ../tools/local-first/derivation-identity.nix;
   inspect = import ./source-qualification-metadata.nix;
@@ -34,7 +36,9 @@ let
       narHashContext = builtins.getContext nixpkgsNarHash;
       evaluatorVersion = builtins.nixVersion;
       pinnedNixVersion = pkgs.nix.version;
-      nixpkgs = pkgs.path;
+      # toJSON ingests path values. Keep the original flake input's outPath
+      # string, so importing it and interpolating pkgs.path does not copy twice.
+      nixpkgs = nixpkgsSource;
       system = pkgs.stdenv.hostPlatform.system;
       expected = comparisonIdentity library;
       expectedMetadata = inspect library;
@@ -42,7 +46,7 @@ let
       yarn = comparisonIdentity library.yarn-freestanding;
     }
   );
-  closure = pkgs.closureInfo { rootPaths = [ pkgs.path ]; };
+  closure = pkgs.closureInfo { rootPaths = [ nixpkgsSource ]; };
 in
 pkgs.runCommand "quixos-protocol-source-check"
   {