Document that StageFixtureManipulator should not be used in build tests

I don't we need to work on this now since our focus is core-mvp issues

I just tried to switch \Drupal\Tests\package_manager\Build\PackageInstallTest to use (Active)FixtureManipulator but failed because it started using \Drupal::service() … which is not available in a build test. So I think the point is pretty much moot here? 😅

Although … I think it would totally be possible at this point, and that'd allow us to remove the last on-disk fixtures 😇

Assigning to myself to see if we can close this.

Also have to decide if we can't if it should be added back to the roadmap