Ensure a stashedFile parameter is retained on a Jenkins restart while in the queue

[mawinter69]

Previously the temporary file was deleted when the job was waiting in the queue and Jenkins was restarted.

fixes #254

Testing done

Added a unit test which fails without this fix.
Manually verified that the temporary folder with the file is deleted when cancelling the entry from the queue

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[jglick]

compare jenkinsci/jenkins#11081

[jglick]

This will work in a test and may work in certain server environments, but will not work in general, for example when running Jenkins as a StatefulSet in Kubernetes with $JENKINS_HOME mounted to a PersistentVolumeClaim: /tmp will be thrown out and recreated on each restart.