We received a bug report that Codex CLI crashes when an env var contains
a non-ASCII character, or more specifically, cannot be decoded as UTF-8:
```shell
$ RUST_BACKTRACE=full RÖDBURK=1 codex
thread '<unnamed>' panicked at library/std/src/env.rs:162:57:
called `Result::unwrap()` on an `Err` value: "RÃ\xB6DBURK"
stack backtrace:
0: 0x101905c18 - __mh_execute_header
1: 0x1012bd76c - __mh_execute_header
2: 0x1019050e4 - __mh_execute_header
3: 0x101905ad8 - __mh_execute_header
4: 0x101905874 - __mh_execute_header
5: 0x101904f38 - __mh_execute_header
6: 0x1019347bc - __mh_execute_header
7: 0x10193472c - __mh_execute_header
8: 0x101937884 - __mh_execute_header
9: 0x101b3bcd0 - __mh_execute_header
10: 0x101b3c0bc - __mh_execute_header
11: 0x101927a20 - __mh_execute_header
12: 0x1005c58d8 - __mh_execute_header
thread '<unnamed>' panicked at library/core/src/panicking.rs:225:5:
panic in a function that cannot unwind
stack backtrace:
0: 0x101905c18 - __mh_execute_header
1: 0x1012bd76c - __mh_execute_header
2: 0x1019050e4 - __mh_execute_header
3: 0x101905ad8 - __mh_execute_header
4: 0x101905874 - __mh_execute_header
5: 0x101904f38 - __mh_execute_header
6: 0x101934794 - __mh_execute_header
7: 0x10193472c - __mh_execute_header
8: 0x101937884 - __mh_execute_header
9: 0x101b3c144 - __mh_execute_header
10: 0x101b3c1a0 - __mh_execute_header
11: 0x101b3c158 - __mh_execute_header
12: 0x1005c5ef8 - __mh_execute_header
thread caused non-unwinding panic. aborting.
```
I discovered I could reproduce this on a release build, but not a dev
build, so between that and the unhelpful stack trace, my mind went to
the pre-`main()` logic we run in prod builds. Sure enough, we were
operating on `std::env::vars()` instead of `std::env::vars_os()`, which
is why the non-UTF-8 environment variable was causing an issue.
This PR updates the logic to use `std::env::vars_os()` and adds a unit
test.
And to be extra sure, I also verified the fix works with a local release
build:
```
$ cargo build --bin codex --release
$ RÖDBURK=1 ./target/release/codex --version
codex-cli 0.0.0
```
06704b1a0f