net/gitlab-agent: Update to 13.12.1.

Required for gitlab-ce 13.12.
The port was removed from deps of gitlab-ce as it is currently not buildable.
Reason is, we have deps that are not existing.
It was reported here:
https://github.com/DataDog/dd-trace-go/issues/929
https://gitlab.com/gitlab-org/cluster-integration/gitlab-agent/-/issues/125

The moment it is fixed upstream, port is again added as dep to gitlab-ce.