gitlab-ci.yml: Fix aggregation logic

After the deployment of the new .workspace job and its application to
the aggregate-docs job, the job was subtly broken. The initial working
directory that the script was executing in was no longer the docs
directory but the root of the workspace.

Fix this by adjusting the logic slightly, this will also make the
rename of the aggregated git repository easier.

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