Gradle Publish fails with Gradle 5

cc: alanmyrvold kenn

:beam-sdks-java-bom:signMavenJavaPublication task fails with an obscure error: (https://scans.gradle.com/s/mcbb4axlx6agy/failure?openFailures=WzBd&openStackTraces=WzFd#top=0):

Duplicate key pom-default.xml.asc:xml.asc:asc:null (attempted merging values Signature pom-default.xml.asc:xml.asc:asc:null and Signature pom-default.xml.asc:xml.asc:asc:null)

Downgrading to Gradle 4 by reverting https://github.com/apache/beam/commit/cadb6f7fabc6faedc6037104338306688f17652f works.