source-and-tags: fix default.nix

Tested by observing that the following command produces a correctly
looking TAGS file:

$ nix-shell -E 'let pkgs = import ./. {}; in pkgs.myEnvFun {name = "myEnvFun-htags-test"; buildInputs = [(pkgs.sourceAndTags.sourceWithTagsDerivation (pkgs.sourceAndTags.addHasktagsTaggingInfo pkgs.haskellPackages.scientific).passthru.sourceWithTags)];}' --run 'cat $TAG_FILES'