Automatically update flake inputs of demos

[ibizaman]

The GitHub job to update the root flake inputs should be extended to also update the demos.

There might be an issue because the demos have their selfhostblocks input URI pointing to the GitHub repository directly. That will of course not have the update just done by the GitHub job. So we'll need to run the following command for each demo but not commit that:

nix flake lock --override-input selfhostblocks ../.. --update-input selfhostblocks

Now that I'm writing this, maybe the above command should instead live inside the GitHub job testing the demos?