Fix includeBuild syntax so it works with grailsPublish

[jdaugherty]

@matrei & I noticed that the tests in the test project fail to run when the project version is considered a release version. The error it gives is:

 A problem occurred evaluating project ':geb'.
 > Failed to apply plugin class 'io.github.gradlenexus.publishplugin.NexusPublishPlugin'.
    > Cannot run Project.afterEvaluate(Action) when the project is already evaluated.

The existing include syntax in the build causes this. After adding end to end testing for the grails-gradle-plugin (https://github.com/jdaugherty/grails-gradle-plugin/actions/runs/12404566767), I was able to isolate this and discovered there's also a includeBuild syntax.

Switching to the includeBuild syntax fixes the subproject so it works regardless of the geb project's version.

[matrei]

Awesome!