Only sign + stage artifacts meant to be deployed

Even with the 'release' profile, it is not necessary to sign each
artifact, because only the ones to be published on Maven Central need
signatures.

Similarly, make Nexus staging deployment to Sonatype OSSRH dependent on
'maven.deploy.skip' and skip staging for non-public artifacts.

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

pom.xml

@@ -326,6 +326,10 @@
 							<goals>
 								<goal>sign</goal>
 							</goals>
+							<configuration>
+								<!-- Only sign artifacts which are also meant to be deployed -->
+								<skip>${maven.deploy.skip}</skip>
+							</configuration>
 						</execution>
 					</executions>
 				</plugin>
@@ -355,6 +359,8 @@
 						<serverId>ossrh</serverId>
 						<nexusUrl>https://oss.sonatype.org/</nexusUrl>
 						<autoReleaseAfterClose>false</autoReleaseAfterClose>
+						<!-- Only stage artifacts to which are also meant to be deployed -->
+						<skipStaging>${maven.deploy.skip}</skipStaging>
 					</configuration>
 				</plugin>
 				<plugin>