c_char depends on platform, so clippy will complain
on linux when it passes on macOS

- Don't do container image CI on forks. (We shouldn't be pushing
  container images to ghcr.io from forks!)

- Remove the `if` condition in `deploy_guide.yml`. It has no effct
  because there is an `on: push: branches: - main` condition higher up
  doing the same thing (and in the more typical way).

(These changes were supposed to be in #268, but I failed to commit the
changes before pushing, sigh.)

By using `rust-cuda` instead. URLs are case-insensitive so this should
have no visible effect.

By using `Rust CUDA` instead.

By using `rust-cuda` instead. This name is used as the WORKDIR, so the
exact value shouldn't matter.