ci/github: add missing 'CI_JOB_IMAGE' env variable

ci/github: add missing 'CI_JOB_IMAGE' env variable

The CI setups of GitLab and GitHub use a common dependency management
script 'ci/install-dependencies.sh'. The script install the necessary
packages based on a combination of the "$distro" and "$jobname" env
variables.

The "$distro" variable is derived from the "CI_JOB_IMAGE" env variable
set by the CI configs. In the GitHub CI config, some of the jobs are
missing this variable. For the 'Documentation' job which depends on
'meson' being installed, this raises an error since the 'meson'
dependency is never installed.

Fix this by adding the 'CI_JOB_IMAGE' variable to all missing jobs. We
don't add it the windows jobs, since they manager their dependency as
part of the CI config and no further dependency management is needed.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>