gitlab-ci.yml: use OHOS_CI_GIT_REPO_PATH

With the improved .workspace and .build jobs merged into the manifest
repository [1], we can now set OHOS_CI_GIT_REPO_PATH and remove the
custom code that was necessary to implement this functionality before.

[1] https://git.ostc-eu.org/OSTC/OHOS/manifest/-/merge_requests/53



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