.gitlab-ci.yml, Makefile: put build artifacts in $CI_PROJECT_DIR

GitLab CI system has a constraint that requires all artifact files to be
placed in a sub-directory of $CI_PROJECT_DIR - aka the place where the
project is automatically checked out by the GitLab runner. Since our
project is manifest-based, we use a temporary scratch directory where
the entire structure is created.

Override `make` invocation, so that it builds the project from the
scratch directory into a build sub-directory of CI_PROJECT_DIR.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@huawei.com>