hack: verify builds are installable

[ahmetb]

This script tests if a build is installable (as documented in the readme)
from its tarball+yaml file.

I felt like adding this as extra confidence on all the refactoring we're doing,
and I know we have integration tests that cover this. Since installation is
almost like our API, I thought it would be good to test.

[codecov-io]

Codecov Report

Merging #262 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #262   +/-   ##
=======================================
  Coverage   53.76%   53.76%           
=======================================
  Files          18       18           
  Lines         863      863           
=======================================
  Hits          464      464           
  Misses        342      342           
  Partials       57       57

---

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[corneliusweig]

/approve
/lgtm
I hope that the number of shell scripts does not continue to grow at the same pace 😉

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ahmetb, corneliusweig

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[ahmetb]

I hope that the number of shell scripts does not continue to grow at the same pace 😉

HAHA I know. But as I was working on some new stuff I wasn't sure if I can install krew locally. Now I know the builds can install themselves. :P