Please enable https://ci.eclipse.org/windowbuilder/ for PGP signing

Summary

WindowBuilder needs to PGP sign the jars it consumes directly from Maven:

https://github.com/eclipse/windowbuilder/issues/302

This PR enables that:

https://github.com/eclipse/windowbuilder/pull/303

But first we need a webmaster to install the project's PGP key on the CI server as a credential/secret for that to work.