lib.customisation: inherit unsafeGetAttrsPos in let-in (#446761)

lib/customisation.nix

···
 let
   inherit (builtins)
     intersectAttrs
+    unsafeGetAttrPos
     ;
   inherit (lib)
     functionArgs
···
       errorForArg =
         arg:
         let
-          loc = builtins.unsafeGetAttrPos arg fargs;
+          loc = unsafeGetAttrPos arg fargs;
         in
         "Function called without required argument \"${arg}\" at "
         + "${loc.file}:${toString loc.line}${prettySuggestions (getSuggestions arg)}";