Return Builds.drvpath in the JSON API

E.g.

curl -H 'Accept: application/json' http://127.0.0.4/build/3772858 | jq -r .drvpath /nix/store/97vla79lavlcsk8milskr06rfm34bskx-patchelf-0.8pre843_f4522f4.drv

Created by  Eelco Dolstra  on September 12, 2017
I7QE7XGKQPKXIGLFLWX7GLZWCZYDQSHELEBLPWORJZKJ2YDZAC3QC
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