ci: set CI_JOB_IMAGE for coverity job

ci: set CI_JOB_IMAGE for coverity job

The main GitHub Actions workflow switched away from the "$distro"
variable in b133d3071a (github: simplify computation of the job's
distro, 2025-01-10). Since the Coverity job also depends on our
ci/install-dependencies.sh script, it needs to likewise set CI_JOB_IMAGE
to find the correct dependencies (without this patch, we don't install
curl and the build fails).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>