deploy github runners for ROOT in the dyngpu cluster

Add a flavor for ROOT runners in https://gitlab.cern.ch/kubernetes/automation/releases/kops-github-arc.

These should run in the DynGPU cluster, not the NGT one.

• AppID: 2153099
• InstallationID: 90957385
• PrivateKey: (in vault kv/kubernetes/services/kops-github-arc/root/github)
• Test project: https://github.com/root-project/NGT-CI-Test