Set CSI hostpath driver version to master if canary run

[RaunakShah]

Even if CSI_PROW_HOSTPATH_CANARY is set to canary, the prow job runs with CSI hostpath driver version set to the default (currently v1.8.0) This PR sets it to master for canary runs.

Example prow run with the issue - https://prow.k8s.io/log?container=test&id=1606218480988721152&job=pull-kubernetes-csi-external-provisioner-canary

Set CSI hostpath driver version to master for canary prow jobs

[k8s-ci-robot]

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[pohly]

Why can't a canary job that sets CSI_PROW_DRIVER_CANARY not also set CSI_PROW_DRIVER_VERSION=master?

This change doesn't seem necessary.