By default, Oomph will use the master version at: https://raw.githubusercontent.com/eclipse/buildship/master/buildship.setup.
The installer can be instructed to use a different version (e.g. via eclipse-inst.ini):
-Doomph.redirection.buildship=https://raw.githubusercontent.com/eclipse/buildship/master/buildship.setup->file:/<path to buildship checkout>/buildship.setup Windows has no eclipse-inst.ini, do this instead:
.\eclipse-inst-jre-win64.exe -vmargs '-Doomph.redirection.buildship=https://raw.githubusercontent.com/eclipse/buildship/master/buildship.setup->file:/<path to buildship checkout>/buildship.setup'
Replace <path to buildship checkout> with the location of your Buildship checkout.
On the variables page, enter the URL of your fork, https://github.com/<user>/buildship, and the branch you want to checkout. This step can be skipped, if you only want to test changes to your buildship.setup (the local version will be used).
To redirect Eclipse to use the local buildship.setup, use -Dbuildship.oomphtest.enabled=true. It will add another variable buildship.oomphtest.setupLocation. Set it to <path to buildship checkout>/buildship.setup.
Without this configuration, the new Eclipse installation, will perform the setup tasks using the buildship.setup from the update site.