pkgs.path: Avoid copying when used via flake

flake.nix

@@ -62,7 +62,7 @@
         }).nixos.manual.x86_64-linux;
       };
 
-      legacyPackages = forAllSystems (system: import ./. { inherit system; });
+      legacyPackages = forAllSystems (system: import ./. { inherit system; config.path = self.outPath; });
 
       nixosModules = {
         notDetected = import ./nixos/modules/installer/scan/not-detected.nix;

pkgs/top-level/all-packages.nix

@@ -67,7 +67,7 @@ with pkgs;
   clangStdenvNoLibs = mkStdenvNoLibs clangStdenv;
 
   # For convenience, allow callers to get the path to Nixpkgs.
-  path = ../..;
+  path = config.path;
 
 
   ### Helper functions.

pkgs/top-level/config.nix

@@ -32,6 +32,20 @@ let
       feature = "run <literal>checkPhase</literal> by default";
     };
 
+    path = mkOption {
+      type = types.path;
+      default = ../..;
+      defaultText = lib.literalDocBook "a path expression";
+      internal = true;
+      description = ''
+        A reference to Nixpkgs' own sources.
+
+        This is overridable in order to avoid copying sources unnecessarily,
+        as a path expression that references a store path will not short-circuit
+        to the store path itself, but copy the store path instead.
+      '';
+    };
+
   };
 
 in {