Pax-Exam should respect the maven.repo.local system property

When running Maven builds, CI containers can be configured to use a different local repository, to better isolate concurrent builds and also to ensure that builds are reproducible. Jenkins does this usually via the maven.repo.local system property. Pax-Exam does not support it out of the box, so I need to work around it using code like

String localRepo = System.getProperty("maven.repo.local", "");
// other stuff...
when(localRepo.length() > 0).useOptions(
    systemProperty("org.ops4j.pax.url.mvn.localRepository").value(localRepo)
)

It would be good if Pax-Exam would do this out of the box. The error message when I fail to do this is quite confusing, and the error is hard to track down:

java.io.IOException: Error resolving artifact org.apache.sling:org.apache.sling.hc.core:jar:0.0.1-SNAPSHOT: Could not find artifact org.apache.sling:org.apache.sling.hc.core:jar:0.0.1-SNAPSHOT in central (http://repo1.maven.org/maven2/)