commands::build::out::tests::unix::test_copy_file fails on macOS

I don't know why this is misbehaving on my machine.

```
---- commands::build::out::tests::unix::test_copy_file stdout ----

thread 'commands::build::out::tests::unix::test_copy_file' panicked at app/buck2_client/src/commands/build/out.rs:556:13:
assertion failed: `Ok(Some(ExitStatus(unix_wait_status(9))))` does not match `Ok(None)`
stack backtrace:
0: rust_begin_unwind
at /rustc/9cd60bd2ccc41bc898d2ad86728f14035d2df72d/library/std/src/panicking.rs:695:5
1: core::panicking::panic_fmt
at /rustc/9cd60bd2ccc41bc898d2ad86728f14035d2df72d/library/core/src/panicking.rs:75:14
2: buck2_client::commands::build::out::tests::unix::test_copy_file::{{closure}}
at ./src/commands/build/out.rs:556:13
3: as core::future::future::Future>::poll
at /Users/jade/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/tracing-0.1.41/src/instrument.rs:321:9
4: tokio::runtime::scheduler::current_thread::CoreGuard::block_on::{{closure}}::{{closure}}::{{closure}}
at /Users/jade/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/tokio-1.44.2/src/runtime/scheduler/current_thread/mod.rs:733:54
5: tokio::task::coop::with_budget
at /Users/jade/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/tokio-1.44.2/src/task/coop/mod.rs:167:5
6: tokio::task::coop::budget
at /Users/jade/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/tokio-1.44.2/src/task/coop/mod.rs:133:5
7: tokio::runtime::scheduler::current_thread::CoreGuard::block_on::{{closure}}::{{closure}}
at /Users/jade/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/tokio-1.44.2/src/runtime/scheduler/current_thread/mod.rs:733:25
8: tokio::runtime::scheduler::current_thread::Context::enter
at /Users/jade/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/tokio-1.44.2/src/runtime/scheduler/current_thread/mod.rs:432:19
9: tokio::runtime::scheduler::current_thread::CoreGuard::block_on::{{closure}}
at /Users/jade/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/tokio-1.44.2/src/runtime/scheduler/current_thread/mod.rs:732:36
10: tokio::runtime::scheduler::current_thread::CoreGuard::enter::{{closure}}
at /Users/jade/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/tokio-1.44.2/src/runtime/scheduler/current_thread/mod.rs:820:68
11: tokio::runtime::context::scoped::Scoped::set
at /Users/jade/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/tokio-1.44.2/src/runtime/context/scoped.rs:40:9
12: tokio::runtime::context::set_scheduler::{{closure}}
at /Users/jade/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/tokio-1.44.2/src/runtime/context.rs:180:26
13: std::thread::local::LocalKey::try_with
at /Users/jade/.rustup/toolchains/nightly-2025-02-16-aarch64-apple-darwin/lib/rustlib/src/rust/library/std/src/thread/local.rs:310:12
14: std::thread::local::LocalKey::with
at /Users/jade/.rustup/toolchains/nightly-2025-02-16-aarch64-apple-darwin/lib/rustlib/src/rust/library/std/src/thread/local.rs:274:15
15: tokio::runtime::context::set_scheduler
at /Users/jade/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/tokio-1.44.2/src/runtime/context.rs:180:9
16: tokio::runtime::scheduler::current_thread::CoreGuard::enter
at /Users/jade/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/tokio-1.44.2/src/runtime/scheduler/current_thread/mod.rs:820:27
17: tokio::runtime::scheduler::current_thread::CoreGuard::block_on
at /Users/jade/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/tokio-1.44.2/src/runtime/scheduler/current_thread/mod.rs:720:19
18: tokio::runtime::scheduler::current_thread::CurrentThread::block_on::{{closure}}
at /Users/jade/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/tokio-1.44.2/src/runtime/scheduler/current_thread/mod.rs:200:28
19: tokio::runtime::context::runtime::enter_runtime
at /Users/jade/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/tokio-1.44.2/src/runtime/context/runtime.rs:65:16
20: tokio::runtime::scheduler::current_thread::CurrentThread::block_on
at /Users/jade/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/tokio-1.44.2/src/runtime/scheduler/current_thread/mod.rs:188:9
21: tokio::runtime::runtime::Runtime::block_on_inner
at /Users/jade/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/tokio-1.44.2/src/runtime/runtime.rs:368:47
22: tokio::runtime::runtime::Runtime::block_on
at /Users/jade/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/tokio-1.44.2/src/runtime/runtime.rs:342:13
23: buck2_client::commands::build::out::tests::unix::test_copy_file
at ./src/commands/build/out.rs:558:13
24: buck2_client::commands::build::out::tests::unix::test_copy_file::{{closure}}
at ./src/commands/build/out.rs:533:38
25: core::ops::function::FnOnce::call_once
at /Users/jade/.rustup/toolchains/nightly-2025-02-16-aarch64-apple-darwin/lib/rustlib/src/rust/library/core/src/ops/function.rs:250:5
26: core::ops::function::FnOnce::call_once
at /rustc/9cd60bd2ccc41bc898d2ad86728f14035d2df72d/library/core/src/ops/function.rs:250:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

failures:
commands::build::out::tests::unix::test_copy_file

test result: FAILED. 20 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.03s
```

Version: 68384d4890301420751de5b9d2a00cd7e37925fb

Environment:
- macOS 15.4.1
- rustup 1.27.1 from nixpkgs
- *not* using the nix shell in buck2, just running `cargo test` directly in the root of the repo.