Do not use multi-threaded maven build for plugins.
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.

vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa