Upload log files to the right location

We were mixing up builds and steps. So for example

https://cache.nixos.org/log/2w66a98iqbjdppc5s2b8qvhi3gprvy45-freecell-solver-4.8.0.drv

at the moment contains the log for /nix/store/442r9d5ihbcpgq8q9dhijhvhlmplzp96-perl-namespace-autoclean-0.28.drv because the latter is a step in http://hydra.nixos.org/build/51300420. Oops.

Created by  Eelco Dolstra  on April 6, 2017
RNJILKTWUYDBY2AX22BRVYFLT3FIPKEXAKGOAW3PA54XBQDENULQC
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