Do not use multi-threaded maven build for plugins.
Parallel build of plugins with the same groupId is unsafe, as
each thread updates the maven-metadata.xml file without any
locking.
@@ -74,7 +74,7 @@ mvn_install -N
# and build plugins first:
echo "Building Vespa Maven plugins."
-mvn_install --threads 1.5C -f maven-plugins/pom.xml
+mvn_install -f maven-plugins/pom.xml
# now everything else should just work with normal maven dependency resolution:
