Add shellcheck linter

What does this MR do?

Adds the shellcheck linter to the make lint target and includes the output in the Code Climate artifact so that the static QA job will report it.

Why was this MR needed?

See #9524

What's the best way to test this MR?

See a failed pipeline and test it on the command line by reverting some of the fixes made and then running make lint.

You can see the report in the MR and noticed that it includes reports from both golangci-lint and shellcheck, meaning they were correctly merged in the lint script:

Is there anything the reviewer needs to take into account?

• shellcheck is GPL v3 which is one of our unacceptable licenses, there is currently an issue for this in https://gitlab.com/gitlab-com/legal-and-compliance/-/issues/43. There is a meeting planned with Legal on June 26th.
• We are reusing the image created for !2224 (merged) instead of adding the linters to the CI image.

What are the relevant issue numbers?

Closes #9524