local plugin in then clause too

[znz]

Summary

I think plugin variable should be local not only in else clause but then clause too.

[Stratus3D]

Thanks for the PR @znz ! These changes look good, however the commit is failing the conventional commit check (https://github.com/asdf-vm/asdf/runs/5987076283?check_suite_focus=true). See https://www.conventionalcommits.org/en/v1.0.0/ for proper formatting. If you are unable to amend your comment message let me know and I can amend it for you.

[Stratus3D]

Closed in favor of #1203