gitlab-ci: do not wait for all jobs from "dependencies" stage.
The GNU/Linux builds should start as soon as the Linux dependencies are built. There is no need to wait for the Windows dependencies (and reciprocally of course). This should make for much faster CI total duration (with current configuration). Note: this "needs" keyword is quite a recent feature since gitlab 12.2, 3 months ago: https://docs.gitlab.com/ee/ci/yaml/#needs