make build-windows-k8s fails after go mod changes

[mboersma]

Describe the bug
scripts/build-windows-k8s.sh contains many references to $GOPATH which don't work any longer after switching to go modules.

Steps To Reproduce
Be careful in testing, becauuse if you have sufficient access this will create a zip file in our staging blob store that should be cleaned up.
It's usually run via a make target command like:

make build-windows-k8s K8S_VERSION=1.2.3 PATCH_VERSION=1

Expected behavior

AKS Engine version
master

Kubernetes version
N/A

Additional context
As an easy workaround for now, run the script from aks-engine located in $GOPATH.

[stale]

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