ci/lib: do not interpret escape sequences in `group ()` arguments

ci/lib: do not interpret escape sequences in `group ()` arguments

We use printf to set up sections with GitLab CI, which requires us to
print a bunch of escape sequences via printf. The group name is
controlled by the user and is expanded directly into the formatting
string, which may cause problems in case the argument contains escape
sequences or formatting directives.

Fix this potential issue by using formatting directives to pass variable
data.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>