bump to a version of ZKAPAuthorizer that (a) works and (b) has new Tahoe-LAFS

nixos/pkgs/privatestorage/repo.json

 {
   "owner": "PrivateStorageio",
-  "branch": "un-mach",
+  "branch": "458.update-tahoe-lafs",
   "repo": "ZKAPAuthorizer",
-  "rev": "cc86cd76cfa4daf6937d63efc2559529af2c5cd5",
+  "rev": "6f8d67c81cdc6de2f52e0a699a077b43232a0589",
   "outputHashAlgo": "sha512",
-  "outputHash": "0v6jcg7mphizkfhvcwcd1p68rm9rmi54hwakarmx3kjwalv4azjc2h9ylq8ziqpziyyymnh4b5yzkjxcbqbv00pb0szf2iqykhw8lcn"
+  "outputHash": "3bg882wlm0bn23xazal81mzac63svg66gcbrabvzqyin98jrwlimk5n64hrdiywiw954g7srpdr1g9f1y4p79vbpnkfkrv7sa108aa4"
 }
\ No newline at end of file