Pin minikube to 1.25.2

Version 1.26.0 fails with the error:

```
X Exiting due to RUNTIME_ENABLE: sudo systemctl enable cri-docker.socket: exit status 1
stdout:

stderr:
Failed to enable unit: Unit file cri-docker.socket does not exist.
```

See:
https://github.com/kubernetes/minikube/issues/14403
https://github.com/kubernetes/minikube/issues/14397

Change-Id: I3f65d75dc87f26eb350256779c120423b52d2e63

.zuul.yaml

@@ -7,6 +7,8 @@
     post-run: playbooks/nodepool-functional-k8s/post.yaml
     required-projects:
       - zuul/nodepool
+    vars:
+      minikube_version: v1.25.2  # NOTE(corvus): https://github.com/kubernetes/minikube/issues/14403 https://github.com/kubernetes/minikube/issues/14397
 
 - job:
     description: |