[JENKINS-68514] Use plugin classLoader to access JAXBContext

[alecharp]

See JENKINS-68514.
This is a followup to #61.

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• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[basil]

CC @ivaivalous

[basil]

@ivaivalous Note that the CI failures are due to the fact that the changes to Jenkinsfile are not being executed in this PR build due to the fact that @alecharp does not have write access to this repository. You can confirm that the changes in this PR pass CI by re-submitting the PR with Jenkinsfile changes under your own GitHub username, which has write access to this repository and which will execute the Jenkinsfile changes.

[basil]

Thanks!

[basil]

Released in 1.26.