Use artifact caching proxy for acceptance-test-harness

[lemeurherve]

This PR activates the use of artifact caching proxy for this repository.

Closes #963

Ref: jenkins-infra/helpdesk#3481 (comment) & jenkins-infra/helpdesk#2752

Testing done

Successfully ran a replay with the no-tranfer-progress option removed from run.sh to check the use of the artifact caching proxy: https://ci.jenkins.io/job/Core/job/acceptance-test-harness/job/PR-1279/4/

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6. Ensure you have provided tests - that demonstrates feature works or fixes the issue

[basil]

Optional feedback for your consideration: Might be desirable to do a few more consecutive test runs (e.g., 3-5 runs) to truly convince ourselves that we are not adding any (new) flakiness? (Note: This repository is already flaky for non-artifact-caching proxy reasons; the only concern is not to make it more flaky, so a reference set of 3-5 test runs without this PR might be needed for a fair apples-to-apples comparison.)

[lemeurherve]

@basil thanks for the feedback! In addition to the initial run without -ntp mvn argument to confirm the use of ACP, I ran 3 consecutive builds from 71ade07, then 2 builds in parallel (same commit), removing disableConcurrentBuilds from these last replays to push a little bit more the load on ACP, with success:

This repository is already flaky for non-artifact-caching proxy reasons; the only concern is not to make it more flaky, a reference set of 3-5 test runs without this PR might be needed

As all test runs from this PR finished without issue, do you think running a reference set of test runs is still desirable or could the existing ones be considered as conclusive?

[basil]

I think we can consider the existing runs as conclusive. Thanks!