gitlab-ci: rename igt-* container to build-*

igt-* better suits for containers which contain IGT inside, while
build-* is better for containers which are used to build IGT.

Reviewed-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Signed-off-by: Oleg Vasilev <oleg.vasilev@intel.com>