Replace our `setup*.sh` files with pixi

- `scripts/setup_dev.sh`
- `scripts/setup_web.sh`
- `scripts/setup.sh`
- `ci_docker/Dockerfile`

First part of this:
- [x] https://github.com/rerun-io/rerun/pull/4302

There is still quite a lot of work to do, and it comes down to us wanting to maintain two workflows:
- Building `rerun` from source
- Contributing to `rerun`

Where the former should not require `pixi`.