ci: Check install and airgap scripts in CI [RHIDP 5162]

[rm3l]

Description

This introduces ShellCheck for checking our scripts (including the CI and airgap ones) in CI.
It leverages https://github.com/redhat-plumbers-in-action/differential-shellcheck

Which issue(s) does this PR fix or relate to

• Fixes https://issues.redhat.com/browse/RHIDP-5162

PR acceptance criteria

• Tests
• Documentation

How to test changes / Special notes to the reviewer

[openshift-ci]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[github-advanced-security]

This pull request sets up GitHub code scanning for this repository. Once the scans have completed and the checks have passed, the analysis results for this pull request branch will appear on this overview. Once you merge this pull request, the 'Security' tab will show more code scanning analysis results (for example, for the default branch). Depending on your configuration and choice of analysis tool, future pull requests will be annotated with code scanning analysis results. For more information about GitHub code scanning, check out the documentation.

[github-advanced-security]

ShellCheck found more than 20 potential problems in the proposed changes. Check the Files changed tab for more details.