ci(lint): add shell linter - Differential ShellCheck

[jamacku]

Description

Addition of a new job in CI workflow that scans all shell scripts in the repo using ShellCheck and reports only newly added defects.

Differential ShellCheck is a GitHub action that performs differential ShellCheck scans on shell scripts changed via PR and reports results directly in PR.

It produces reports in SARIF format. GitHub understands this format and is able to display it nicely as a PR comment, and on the Files Changed tab, please see below.

Documentation is available at @redhat-plumbers-in-action/differential-shellcheck.

Screenshots

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[aireilly]

Thanks for the PR. Any particular reason why you are targeting this repo with this update? You've not contributed here before.

[jamacku]

I just came across your org and this repo, and I noticed that you have some shell scripts and don't run ShellCheck in your CI.

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[themr0c]

Seems a duplicate with https://github.com/redhat-documentation/vale-at-red-hat/blob/main/.github/workflows/validate-scripts.yml

[aireilly]

Not required