Fix undercoverage not existing in older branches

Thong Kuahrequested to merge
fix_undercoverage_not_existing_in_old_branches into master

What does this MR do and why?

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Fixes the issue described in https://gitlab.com/gitlab-org/gitlab/-/issues/346972#note_749898426

We deliberately reset to the MR source branch for comparison. However the undercoverage script, and gem might not be present for older MR source branches. So, as a transitionary measure we skip undercoverage if the script file is not present.

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