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/execpolicy/tests/bad.rs

@@ -1,5 +1,5 @@
-use codex_execpolicy::get_default_policy;
 use codex_execpolicy::NegativeExamplePassedCheck;
+use codex_execpolicy::get_default_policy;
 
 #[test]
 fn verify_everything_in_bad_list_is_rejected() {