Don't use posix_spawn_file_actions_addchdir_np on macOS.

Author: ehuss

Diff for: library/std/src/sys/unix/process/process_unix.rs

@@ -314,10 +314,20 @@ impl Command {
             ) -> libc::c_int
         }
         let addchdir = match self.get_cwd() {
-            Some(cwd) => match posix_spawn_file_actions_addchdir_np.get() {
-                Some(f) => Some((f, cwd)),
-                None => return Ok(None),
-            },
+            Some(cwd) => {
+                if cfg!(target_os = "macos") {
+                    // There is a bug in macOS where a relative executable
+                    // path like "../myprogram" will cause `posix_spawn` to
+                    // successfully launch the program, but erroneously return
+                    // ENOENT when used with posix_spawn_file_actions_addchdir_np
+                    // which was introduced in macOS 10.15.
+                    return Ok(None);
+                }
+                match posix_spawn_file_actions_addchdir_np.get() {
+                    Some(f) => Some((f, cwd)),
+                    None => return Ok(None),
+                }
+            }
             None => None,
         };