symlinkJoin: truly pass the defaulted parameters

The catch is that in nix the @-pattern binds the set that was *explicitly*
passed to the function, i.e. default values are *not* taken into account.

pkgs/build-support/trivial-builders.nix

@@ -49,15 +49,17 @@ rec {
 
   # Create a forest of symlinks to the files in `paths'.
   symlinkJoin =
-    args@{ name
+    args_@{ name
          , paths
          , preferLocalBuild ? true
          , allowSubstitutes ? false
          , postBuild ? ""
          , ...
          }:
-    runCommand name
-      (removeAttrs args [ "name" "postBuild" ])
+    let
+      args = removeAttrs args_ [ "name" "postBuild" ]
+        // { inherit preferLocalBuild allowSubstitutes; }; # pass the defaults
+    in runCommand name args
       ''
         mkdir -p $out
         for i in $paths; do