Reproducible builds

I'm trying to make my [builds reproducible](https://maven.apache.org/guides/mini/guide-reproducible-builds.html) but it is not currently possible with `.nbm` files.
More specifically, the Maven property `project.build.outputTimestamp` is not used by the `nbm-maven-plugin` plugin.

I've tried to find a way in the plugin code but that didn't work.

Any idea?

Start by tracing how the nbm-maven-plugin creates .nbm files and handles archive timestamps, then inspect how Maven's project.build.outputTimestamp could be used there. Done means builds using that property produce reproducible .nbm files; verify by comparing outputs from repeated builds.

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