Mark maven plugins are being thread-safe

[netomi]

If the plugins are using in parallel builds, maven will issue a warning as it cannot determine if the plugings are thread-safe:

09:29:15  [WARNING] *****************************************************************
09:29:15  [WARNING] * Your build is requesting parallel execution, but this         *
09:29:15  [WARNING] * project contains the following plugin(s) that have goals not  *
09:29:15  [WARNING] * marked as thread-safe to support parallel execution.          *
09:29:15  [WARNING] * While this /may/ work fine, please look for plugin updates    *
09:29:15  [WARNING] * and/or request plugins be made thread-safe.                   *
09:29:15  [WARNING] * If reporting an issue, report it against the plugin in        *
09:29:15  [WARNING] * question, not against Apache Maven.                           *
09:29:15  [WARNING] *****************************************************************
09:29:15  [WARNING] The following plugins are not marked as thread-safe in eclipse-platform-parent:
09:29:15  [WARNING]   org.eclipse.cbi.maven.plugins:eclipse-jarsigner-plugin:1.4.3

Generally, all our plugins should be thread-safe (need to double check though) and we should add the threadsafe = true flag to the Mojo annotation.

[netomi]

The flatpak plugin is not threadsafe as it has an instance variable flatpakId that gets modified in the execute method.