Eclipse Amlen builds fail if multiple run in parallel

Summary

Recently a "resourcepack" was added to Eclipse Amlen and now our Jenkins builds attempt to run in parallel - but if two do run at the same time, one usually fails. The symptom of the failed build vary but sometimes there is a cryptic (to me) error: java.nio.channels.ClosedChannelException Also: hudson.remoting.Channel$CallSiteStackTrace: Remote call to JNLP4-connect connection from 10.40.29.81/10.40.29.81:52584

Full error: https://ci.eclipse.org/amlen/job/AmlenBuilds/job/levell-updatecontribbuild/1/console

Our jenkinsfile that runs on each commit is: https://github.com/eclipse/amlen/blob/main/Jenkinsfile

(We've recently expanded what we do in the deploy phase which runs in the jnlp container. We can reduce it, but as I understand it that is the recommended container for copying things up to the download area so reducing it might(?) make the race condition smaller (as the error mentions JNLP) but won't fix it.

Steps to reproduce

Deliver code to two different Amlen branches within 30mins of each other

What is the current bug behaviour?

One build tends to fail.

What is the expected correct behaviour?

Both builds run in parallel

Relevant logs and/or screenshots

https://ci.eclipse.org/amlen/job/AmlenBuilds/job/levell-updatecontribbuild/1/console