[MNGSITE-443] Add setFile to Mojo Cookbook (website)

**[Casper Roubos](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=casper_nl)** opened **[MNGSITE-443](https://issues.apache.org/jira/browse/MNGSITE-443?redirect=false)** and commented

Is it possible to add in the 'my first mojo', that you need to register the output-directory in the artifact with project.getArtifact().setFile(..).

We are using a Jenkins muti-branch pipeline, and we found out that if you not build to install (e.q. process-test-classes) that the build will give an error because the dependency resolution can't find the artifact in the repo (because the first time it is not deployed yet).

Hope that this addition will also others.

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No further details from [MNGSITE-443](https://issues.apache.org/jira/browse/MNGSITE-443?redirect=false)