Fix race between hydra-eval-jobs and hydra-update-gc-roots

If hydra-eval-jobs creates a new root, and hydra-update-gc-roots runs
before hydra-evaluator has had a chance to add the corresponding build
to the database, then hydra-update-gc-roots will remove the root. If
subsequently the Nix garbage collector kicks in, it may remove the
build's .drv file before the build is performed. Since evaluation of
the Nixpkgs and NixOS jobsets nowadays takes a lot of time (e.g. an
hour), the probability of this happening is fairly high.

The quick fix is not to delete roots that are less than a day old. So
long as evaluation doesn't take longer than a day, this should be fine
;-)

Fixes #166.