prow.sh: only run alpha tests for latest Kubernetes release
prow.sh: only run alpha tests for latest Kubernetes release

Commit 7bcee13d79012 added alpha feature gates for Kubernetes 1.19 in
the CSI_PROW_E2E_ALPHA_GATES_LATEST variable based on the comment in
https://github.com/kubernetes-csi/external-provisioner/pull/493#discussion_r502663402
that alpha testing only runs for the latest Kubernetes.

But some components (like external-health-monitor) are configured with
a single Prow job which runs the default set of tests on a stable
Kubernetes release (currently 1.17). Those tests used to include alpha
testing which then broke during Kind cluster startup because the
Kubernetes 1.19 feature gates weren't recognized by 1.17.

The solution is to disable alpha testing for Kubernetes != latest in
the default set of tests.

kubernetes-csi/csi-driver-nfs,kubernetes-csi/csi-driver-nfs,kubernetes-csi/csi-driver-nfs