The image for the "linux-breaking-changes" job has drifted apart across
GitHub and GitLab. Adapt it to use "ubuntu:rolling" on both systems.
With this change there's only one difference remaining: GitHub uses
"ubuntu:focal" for the "linux32" job while GitLab uses "ubuntu:20.04".
These are different names for the same image, so there is no actual
difference here. Adjust GitHub to use the "20.04" tag -- this matches
all the other jobs which use version numbers, and you don't have to
learn Ubuntu's release names by heart.
Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>