symlinkJoin can break (silently) when the passed paths contain symlinks
to directories. This should work now.
Down-side: when lib/tmpfiles.d doesn't exist for some passed package,
the error message is a little less explicit, because we never get
to the postBuild phase (and symlinkJoin doesn't provide a better way):
/nix/store/HASH-NAME/lib/tmpfiles.d: No such file or directory
Also, it seemed pointless to create symlinks for whole package trees
and using only a part of the result (usually very small part).
