Scribunto: double the Lua CPU limit on the job runners

[[w:User:Jackmcbarn]] noticed that scripts that execute under the limit on the
web tier machines can sometimes hit the limit and fail on the job runners, on
account of the increased load on those machines. As a stopgap, double the limit
(raising it from 10s to 20s) on those machines.

Change-Id: I4038ee700f3fd5d6e528fc528ea59930309be7d1