Negative lookups are purged from the DB after a day, at most once per
day. However, for non-"have" lookups (e.g. all except "nix-env
-qas"), negative lookups are ignored after one hour. This is to
ensure that you don't have to wait a day for an operation like
"nix-env -i" to start using new binaries in the cache.
Should probably make this configurable.