Avoid file locks in LauncherTest.remotable

[jglick]

Amending test introduced in #3563 since as noted in #8006 (comment) it can have file locking issues on Windows. Simplified as per jenkinsci/workflow-durable-task-step-plugin@4cf6db4.

Testing done

Passes locally on both Linux and Windows. (Could not reproduce failure on Windows but did not try hard.)

Proposed changelog entries

• N/A

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[MarkEWaite]

Nice!

[NotMyFault]

/label ready-for-merge

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This PR is now ready for merge. We will merge it after ~24 hours if there is no negative feedback.
Please see the merge process documentation for more information about the merge process.
Thanks!