Support obtaining the contents of an evaluation as a channel

E.g. http://hydra.nixos.org/eval/552513/channel

The advantage of such a channel is that all builds come from the same evaluation, and thus are consistent.

Created by  Eelco Dolstra  on April 2, 2012
T273UOB7WVB3NK52EBN65FQR6XP746D3BZM5SOYXEV6NCARIRYOQC
In channels
main
upstream
Change contents