Don't check tmpdir_prefix existence

[stxue1]

Resolves #5310

Changelog Entry

To be copied to the draft changelog by merger:

• Fixed a regression with tmpdir_prefix

Reviewer Checklist

• Make sure it is coming from issues/XXXX-fix-the-thing in the Toil repo, or from an external repo.
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    contrib/admin/test-pr otheruser theirbranchname issues/XXXX-fix-the-thing
    

  • If there is no associated issue, create one.
• Read through the code changes. Make sure that it doesn't have:
  • Addition of trailing whitespace.
  • New variable or member names in camelCase that want to be in snake_case.
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  • New features without tests.
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Merger Checklist

• Make sure the PR passed tests, including the Gitlab tests, for the most recent commit in its branch.
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• Copy its recommended changelog entry to the Draft Changelog.
• Append the issue number in parentheses to the changelog entry.

[adamnovak]

We can merge this by itself, but since we've had to fix this twice now we really need a test. Either this shouldn't actually close the issue, or we should make a new issue for the test we need.

[adamnovak]

Looks good to me!