Much like in
https://github.com/eclipse-equinox/p2/blob/f682eb19414bfcdef31ea498f4b3bffb76166d5[…]bundles/org.eclipse.equinox.p2.tests.discovery/build.properties
and https://github.com/eclipse-equinox/p2/blob/f682eb19414bfcdef31ea498f4b3bffb76166d58/bundles/org.eclipse.equinox.p2.tests/pom.xml#L23
signing inner jars is in general disabled in other places of the build
as it's not enhancing the security (signature for the inner jar is in
the outer one MANIFEST.MF)