Update cargo to 2024 edition (#842)

Some effects of this change:
- New formatting changes across many files. No functionality changes
should occur from that.
- Calls to `set_env` are considered unsafe, since this only happens in
tests we wrap them in `unsafe` blocks

codex-rs/exec/src/main.rs

@@ -1,6 +1,6 @@
 use clap::Parser;
-use codex_exec::run_main;
 use codex_exec::Cli;
+use codex_exec::run_main;
 
 #[tokio::main]
 async fn main() -> anyhow::Result<()> {