Generate a temporary uber-jar in case the target already exists instead of deleting it right away

Currently, before generating the runner JAR, we are checking if it already exists and deleting it. In case `quarkus.package.add-runner-suffix=false`, the uber-jar file name will (typically) be matching the original JAR name. The issue is that the original JAR is used as a content source for the uber-jar. If the original JAR is deleted before the uber-jar is generated than generated uber-jar won't be usable.