Execute Maven plugin goal on parent module, but not on children

As documented in the Plugins section of the pom reference:

Beyond the standard coordinate of groupId:artifactId:version, there are elements which configure the plugin or this builds interaction with it.

• inherited: true or false, whether or not this plugin configuration should apply to POMs which inherit from this one.

So just add <inherited>false</inherited> to the buildnumber-maven-plugin configuration to avoid inheritance in children POMs:

      <plugin>
        <groupId>org.codehaus.mojo</groupId>
        <artifactId>buildnumber-maven-plugin</artifactId>
        <version>1.0-beta-3</version>
        <inherited>false</inherited>
        ...
      </plugin>