Fix shellcheck linter reported issues (!2232) · Merge requests · GitLab.org / gitlab-runner · GitLab

What does this MR do?

Fixes the warnings reported by shellcheck in !2220 (closed)

Why was this MR needed?

To make !2220 (closed) a smaller MR focusing only on introducing the linter.

What's the best way to test this MR?

What are the relevant issue numbers?

Closes #9524