signCordappJar breaks Gradle caching semantic

[remmeier]

There is a a dedicated signCordappJar task that executed after the jar task and modifies the same file. Gradle caching algorithms require for tasks to make use of disjoint output paths in order to clearly track which file is touched by which tasks. It is probably best to add the signature within the Jar task or alternatively create a new signed jar (maybe the fat jar should be another jar anyway).