.oniro-ci,gitlab-ci.yml: add and display CI_ONIRO_MAGIC

Our GitLab pipeline jobs rely on both the .extends mechanism and the
re-definition-in-place mechanism, in order to avoid repetition.
Something is misbehaving, as evidenced by [1] and [2], suggesting that
the way GitLab computes the effective set of jobs in a pipiline is
different from what we understand.

The CI_ONIRO_MAGIC variable is set to the name and location of the job
defining the particular value. By showing the effective value of the
variable, we may gain some insight into what is going on in practice.

[1] eclipse/oniro-core/oniro#613
[2] eclipse/oniro-core/oniro#574



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