Book deployment is broken

[ebfull]

error[E0658]: the `unsafe_op_in_unsafe_fn` lint is unstable
Error:    --> /home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/os_str_bytes-6.0.0/src/lib.rs:159:1
    |
159 | #![forbid(unsafe_op_in_unsafe_fn)]
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: see issue #71668 <https://github.com/rust-lang/rust/issues/71668> for more information

error: aborting due to previous error
Error: aborting due to previous error
For more information about this error, try `rustc --explain E0658`.
error: could not compile `os_str_bytes`
Error: could not compile `os_str_bytes`
To learn more, run the command again with --verbose.
warning: build failed, waiting for other jobs to finish...
Warning: error: failed to compile `mdbook-katex v0.2.10`, intermediate artifacts can be found at `/tmp/cargo-installOt9WtP`

[str4d]

Book deployment is now broken because os_str_bytes requires Rust 1.57, but book compilation uses the same MSRV as our repo (1.56.1). We should fix this by always compiling the book with stable.