hydra-update-gc-roots: Keep more evals

We now keep all unfinished evaluations of a jobset, in addition to the <keepnr> most recent finished evaluations.

The main motivation is to ensure that mirror-{nixos,nixpkgs} work properly: if building an evaluation takes too long, some of its builds may already have been garbage-collected by the time the others finish.

Created by  Eelco Dolstra  on September 18, 2013
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