Enabling base64 token encoding for custom registry authentication

As described here, Kaniko requires a base64 encoded token for auth towards registries that are not the standard GitLab (e.g. registry.cern.ch).

I've verified that such strategy works (i.e. I'm able to authenticate and push to the registry) in this commit (and related pipeline).

If I do not base64 encode, I get permission denied on the push, e.g. this job.