[ci.jenkins.io] Use the jenkins-infra's Artifact Caching Proxy to benefit from cached artifacts

[dduportal]

With #537, the build stability was restored by opting-out of the "ACP" (Artifact Caching Proxy) system on ci.jenkins.io.

The (integration) tests failures are rooted into the resulting settings.xml passed to inner Maven invokation on each IT: on ci.jenkins.io, this src/it/settings.xml file is merged with the infrastructure's custom settings.xml used to benefit from ACP causing unexpected pluginRepository order.

[basil]

Should this issue be transferred to jenkins-infra/helpdesk?

[dduportal]

Should this issue be transferred to jenkins-infra/helpdesk?

I hesitated to be honest. The scope (project maven-hpi-plugin) makes sense to have the issue here, but having it in jenkins-infra/helpdesk would ensure tracking (or at least not being forgotten) by the infra team.

[basil]

OK, maybe we can have two tickets then, each one linking to the other, to cover both cases?