ci-package-builder: Replace custom variable with CI_REPOSITORY_URL
The $CI_REPOSITORY_URL predefined variable provides an authenticated
URL for the current git repository which allows pull and push, just like
our custom $CI_AUTH_PROJECT_URL variable.
However, $CI_REPOSITORY_URL does not require global credentials to be
configured and rather uses the per-job ephemeral auth token, which
means that:
- jobs run with much more limited privileges as they can only write to
the repository on which they run, rather than any repository the
global credentials have access to (everything under the
pkg/group) - there's less configuration needed, as there's no need to use a dedicated bot user and set its credentials in CI variables