nix: point nix edit and ofborf at the original version attribute

This way `nix edit` will just at the location close to the attribute
definition and not the common code.

pkgs/tools/package-management/nix/common.nix

@@ -6,7 +6,7 @@
 , src ? fetchFromGitHub { owner = "NixOS"; repo = "nix"; rev = version; inherit hash; }
 , patches ? [ ]
 , maintainers ? with lib.maintainers; [ eelco lovesegfault artturin ]
-}:
+}@args:
 assert (hash == null) -> (src != null);
 let
   atLeast24 = lib.versionAtLeast version "2.4pre";
@@ -232,6 +232,9 @@ self = stdenv.mkDerivation {
     };
   };
 
+  # point 'nix edit' and ofborg at the file that defines the attribute,
+  # not this common file.
+  pos = builtins.unsafeGetAttrPos "version" args;
   meta = with lib; {
     description = "Powerful package manager that makes package management reliable and reproducible";
     longDescription = ''