Add shell linter github action

[kauana]

Changes

• Add shellcheck linter as a github action (uses knative's reusable style action)
• See this PR for an example of how the action works when someone attempts to add bad shell code

/kind enhancement

Fixes #269

[knative-prow[bot]]

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[krsna-m]

Hey there, sorry for this hanging here for so long. I think the correct solution is to add hack repo to knobots to use the style directive. That way the workflow is managed by the automation system. There is examples of the style reusable action in serving, eventing, etc. that uses this system.

[kauana]

Hey @kvmware, sorry for the wait as well! I believe knative/hack is already added to knobots, but the github actions are currently excluded for this repo.

If that's ok with you, I'm happy to update that so knative/hack can take advance of the style action. Thanks

[krsna-m]

That should work thanks