Use posix_spawn for absolute paths on macOS Currently, on macOS, Rust never uses the fast posix_spawn path if a directory change is requested, due to a bug in Apple's libc. However, the bug is only triggered if the program is a relative path. This PR makes it so that the fast path continues to work if the program is an absolute path or a lone filename. This was an alternative proposed in https://github.com/rust-lang/rust/pull/80537#issue-776674009, and it makes a measurable performance difference in some of my code that spawns thousands of processes.