Previously we would call 'machine-load' once per machine, which was very
costly when there were many machines. Now we arrange to call it only
once on average (when all the machines have the same 'speed' value).
* guix/scripts/offload.scm (random-seed, shuffle): New procedures.
(choose-build-machine)[machines+slots+loads]: Rename to...
[machines+slots]: ... this. Remove load from the tuples therein.
[undecorate]: Adjust accordingly.
[machine-less-loaded-or-faster?]: Remove.
[machine-faster?]: New procedure.
Sort MACHINES+SLOTS according to 'machine-faster?'. Call
'machine-load?' as the last thing.
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