Switch k8s master CI to go1.13.4

[liggitt]

Changes CI for master to run using go1.13.4 (I think)

Prereq for green runs on kubernetes/kubernetes#82809

Once master switches to go1.13.4, other PRs will fail verify checks until kubernetes/kubernetes#82809 merges.

[liggitt]

/cc @BenTheElder

[cblecker]

I'm not sure if we need to bump master/experimental in the krte images too: images/krte/variants.yaml

[BenTheElder]

it shouldn't be strictly necessary currently (we only use bazel with them at the moment) but I'd appreciate it.

KRTE currently only supports testing with kind specifically, but it's more or less kubekins-e2e optimized and dropping support for bootstrap.py, I'm using it to figure out what we can do to clean up the rest of the kubekins jobs someday..

[liggitt]

/hold

[BenTheElder]

/lgtm
/approve
note that we'll need to file an image bump to pick up these after this PR merges (CI will auto build / push them though)

[cblecker]

/hold cancel

[stevekuznetsov]

/hold cancel

[k8s-ci-robot]

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