gitlab-ci: Retry jobs after runner system failures

Up to twice, for a total of 3 attempts maximum.

This will hopefully avoid spurious CI pipeline failures due to
intermittent GitLab/docker infrastructure issues.

Inspired by
mesa/mesa@6140ed3d

Reviewed-by: Eric Anholt <eric@anholt.net>
3 jobs for ci-tweaks in 6 minutes and 30 seconds