Prevent multiple builds with the same (job, outPath) tuple from being added

This happened in a pathological case in Nixpkgs: the "grub" job is evaluated for i686-linux and x86_64-linux, but in the latter case it returns the same derivation as in the former case. So only one build should be added.

Created by  Eelco Dolstra  on April 2, 2012
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