nixos: tarsnap - separate archive cachedirs

Tarsnap locks the cachedir during backup, meaning if you specify
multiple backups with a shared cache that might overlap (for example,
one backup may take an hour), secondary backups will fail. This isn't
very nice behavior for the obvious reasons.

This splits the cache dirs for each archive appropriately. Note that
this will require a rebuild of your archive caches (although if you were
only using one archive for your whole system, you can just move the
directory).

Signed-off-by: Austin Seipp <aseipp@pobox.com>