basename fix for imputation beagle ref panel generation

[mmorgantaylor]

Description

Give your PR a concise yet descriptive title. Please explain the changes you made here. Explain the motivation for making this change. What existing problem does the pull request solve? Mention any issues fixed, addressed, or otherwise related to this pull request, including issue numbers or hard links for issues in other repos. You can delete these instructions once you have written your PR description.

---

Checklist

If you can answer "yes" to the following items, please add a checkmark next to the appropriate checklist item(s) and notify our WARP documentation team by tagging either @ekiernan or @kayleemathews in a comment on this PR.

• [ ] Did you add inputs, outputs, or tasks to a workflow?
• [ ] Did you modify, delete or move: file paths, file names, input names, output names, or task names?
• [ ] If you made a changelog update, did you update the pipeline version number?

[dsde-jenkins]

Can one of the admins verify this patch?