pkgsStatic.ghc: fix attr pointing to package set sometimes - #507998

       … while calling the 'throw' builtin
         at /var/lib/ofborg/checkout/repo/38dca4e3aa6bca43ea96d2fcc04e8229/builder/ofborg-core/lib/asserts.nix:50:34:
           49|   # TODO(Profpatsch): add tests that check stderr
           50|   assertMsg = pred: msg: pred || throw msg;
             |                                  ^
           51|

       error: GHC >= 9.6 can't be cross-compiled. If you meant to build a GHC cross-compiler, use `buildPackages`.