gvisor fixes for v1.7.0

[tstromberg]

• /tmp/gvisor must exist before we can copy files into it
• Don't attempt to kubectl apply non-YAML files in an addon

Fixes error:

⚠️ Enabling 'gvisor' returned an error: running callbacks: [sudo test -d /tmp/gvisor && sudo scp -t /tmp/gvisor && sudo touch -d "0001-01-01 00:00:00 +0000" /tmp/gvisor/gvisor-config.toml: Process exited with status 1

and:

⚠️ Enabling 'gvisor' returned an error: running callbacks: [addon apply: sudo KUBECONFIG=/var/lib/minikube/kubeconfig /var/lib/minikube/binaries/v1.17.2/kubectl apply -f /etc/kubernetes/addons/gvisor-pod.yaml -f /etc/kubernetes/addons/gvisor-runtimeclass.yaml -f /tmp/gvisor/gvisor-config.toml: Process exited with status 1

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: tstromberg

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[medyagh]

/ok-to-test

[minikube-pr-bot]

Error: running mkcmp: exit status 1

[codecov-io]

Codecov Report

Merging #6512 into master will decrease coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #6512      +/-   ##
==========================================
- Coverage   37.95%   37.95%   -0.01%     
==========================================
  Files         138      138              
  Lines        8707     8708       +1     
==========================================
  Hits         3305     3305              
- Misses       4985     4986       +1     
  Partials      417      417

Impacted Files	Coverage Δ
pkg/minikube/cluster/start.go	52.33% <ø> (ø)	⬆️
pkg/addons/addons.go	45.67% <0%> (-0.29%)	⬇️