Allow building the local grid with a clean checkout.

It should be possible to build the configuration for the local grid, without having a dirty file in the repository. I often build all three grids locally, either to make sure that they all at least still build after changes, or to ensure that a refactoring I've done hasn't had any impact on the final configuration.

Having to edit morph/grid/local/public-keys/users.nix to build the local grid makes this more difficult.