d/a/gitlab-ci: Update the build dir to reflect the change in ci-buildpackage

Commit infrastructure/apertis-docker-images@b9b045e8 "Use uncommon build directory name" changed the build directory name from build to _build.

infrastructure/apertis-docker-images@b9b045e8

The change needs to be reflected here, as already done in infrastructure/ci-package-builder@cbc8a0de.