Upgrade build images to node.js 12.6 or newer

In gitlab-build-images!207 (merged) we have pinned our node.js version to 12.4.0 because of performance problems with higher versions (see https://gitlab.com/gitlab-org/gitlab-ce/issues/64258#note_189626358). As soon as a newer 12.6.x version is released with the performance problems fixed, we should upgrade.