Fix selective Nexus deployment to Sonatype OSSRH

Fix selective Nexus deployment to Sonatype OSSRH

Other than Maven Deploy, Nexus Staging plugin cannot just be added to
the 'build/plugins' section of the parent POM once and (de-)activated
with a simple property like 'maven.deploy.skip' on a per-module basis.
See also https://issues.sonatype.org/browse/OSSRH-68966. Consequently,
we do not add it to the parent but separately to each module meant to be
published.

Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>