ci: bump venus-lavapipe fraction

It's been running very slowly lately and is the bottleneck for some merges now that no hardware is actively on fire (yay!!!)

See https://gitlab.freedesktop.org/mesa/mesa/-/jobs/59715767 from a recent premerge pipeline.

Bump the fraction to get us under the max job time (soft limit 10min, hard limit 15min - linked job was 21:08 of deqp-runner time plus 1:21 of overhead).