Only pass CI_JOB_TOKEN when using custom images.

Set image_needs_job_token when generating jobs and only pass
CI_JOB_TOKEN when it’s defined. This avoids having to work around
it when CI_JOB_TOKEN is not necessary and the job definition is used
e.g. with lava-submit/lqa which do not support "job" variable.

Signed-off-by: Andrej Shadura <andrew.shadura@collabora.co.uk>