It used to be necessary to override from where the E2E suite came on a
case-by-case basis (initially, testing was using a more recent suite
against an older Kubernetes). This should never become necessary again
and the lack of a specific entry for 1.18 already had the unintended
effect that Kubernetes 1.18 was tested with the suite from master, so
overall it is better to always use the E2E suite which matches
Kubernetes.