merge conflict in dune when config-builder runs on branch

[7yl4r]

The config-build should only build the datasets.xml file for the master branch.
The builder (erroneously) ran on the dune branch once in gh actions, creating a merge conflict when trying to do the automated rebase on the server.

I have updated the action .yml file to explicitly run only on the master branch.

Leaving this issue open here for a little while in case we hit this again, but will close this once enough time has passed to regain confidence in the automated CI setup.