Exit with correct exit code for undercoverage script

What does this MR do and why?

Fixes a regression in scripts/undercoverage not failing CI job correctly (e.g. https://gitlab.com/gitlab-org/gitlab/-/jobs/2744094860)

Copies https://github.com/grodowski/undercover/blob/32e62f66682ee566032b5970437ed2934ef29701/bin/undercover#L12-L16

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