emacsPackages.bpr: fix future preBuild

We conditionally set `preBuild` depending on version. For a new enough
version of the `bpr` package, we would set `preBuild` to the
`previousAttrs` attrset.

This looks like a copy/paste error, so fix it.

This code is currently not reachable: it would need a newer version of
`bpr`, which has had no changes upstream for several years.

pkgs/applications/editors/emacs/elisp-packages/melpa-packages.nix

@@ -897,7 +897,7 @@ let
                     rm --verbose --force test-bpr.el
                   ''
                 else
-                  previousAttrs;
+                  previousAttrs.preBuild or "";
             }
           );