Env variable `WPA_TAG` not exported in `hack/install.sh`

[izackwu]

As environment variable WPA_TAG is not exported in hack/install.sh, when running hack/generate.sh, it will fail to replace the placeholder {{ WPA_TAG }} properly in the deployment template.

       image: practodev/workerpodautoscaler:{{ WPA_TAG }}

The consequence is that the workerpodautoscaler pod will be stuck in status InvalidImageName:

$ k get pods -n kube-system
NAME                                        READY   STATUS             RESTARTS   AGE
workerpodautoscaler-7bbd6f667-z59b6         0/1     InvalidImageName   0          17s