ci: remove aggregated directory from previous runs

The .oniro-repo-workspace job uses GIT_STRATEGY=none, so CI_PROJECT_DIR
is not cleaned up between jobs. As a consequence, the "aggregated"
repository checkout may be present on job startup, thus upsetting
remaining logic.

Remvoe the aggregated repository before starting the build.

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