Test zerocopy-derive when zerocopy's `derive` feature is not enabled

In the past, we've [written code](https://github.com/google/zerocopy/commit/4faad6efe2ad671590958ea7766bfc219c437eee) which breaks this configuration (probably; I haven't actually tested that one because we later made a change which removed that `cfg` for unrelated reasons). We should add a test in CI that tests a crate which depends on zerocopy without the `derive` feature and also depends on zerocopy-derive directly.

One option would be to create a dedicated crate for this purpose with a few basic uses of derives. Another would be to figure out how to compile all of our existing tests in both configurations.