Use Artifact Caching Proxy

[lemeurherve]

This PR reactivates the use of artifact caching proxy as the jitpack.io 3rd party Maven repository has been added to ci.jenkins.io mirror settings exceptions in jenkins-infra/jenkins-infra#2895.

Ref: jenkins-infra/helpdesk#3591

Testing done

Replayed a PR build with this option set to true: https://ci.jenkins.io/job/Plugins/job/ircbot-plugin/job/PR-195/2/console

Finished: SUCCESS

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[lemeurherve]

FYI unrelated to this PR, Java 19 is available on ci.jenkins.io
(cf

ircbot-plugin/Jenkinsfile

Lines 18 to 20 in 268aa79

	// Test the bleeding edge of the compatibility spectrum (i.e., the latest supported Java runtime).
	// see also https://www.jenkins.io/doc/developer/plugin-development/choosing-jenkins-baseline/
	[ platform: 'linux', jdk: '17', jenkins: '2.375.2' ],

)