In the dashboard and on the job page, indicate whether the job appears
in the latest jobset eval. That way, the user gets some indication if
a job has accidentally disappeared (e.g. due to an evaluation error).
Due to the fixed-output derivation hashing scheme, there can be
multiple derivations of the same output path. But build logs are
indexed by derivation path. Thus, we may not be able to find the
log of a build or build step using its derivation. So as a fallback,
Hydra now looks for other derivations with the same output paths.
It's pointless to store these, since Nix knows where the logs are.
Also handle (in fact require) Nix's new log storage scheme. Also some
cleanups in the build page.
