Fix Unix.utimes not working for directories on Win32. (#8796)

Changes

@@ -294,6 +294,10 @@ OCaml 4.12.0
 
 ### Other libraries:
 
+- #8796: On Windows, make Unix.utimes use FILE_FLAG_BACKUP_SEMANTICS flag
+  to allow it to work with directories.
+  (Daniil Baturin, review by Damien Doligez)
+
 * #9206, #9419: update documentation of the threads library;
   deprecate Thread.kill, Thread.wait_read, Thread.wait_write,
   and the whole ThreadUnix module.

otherlibs/win32unix/utimes.c

@@ -56,7 +56,7 @@ CAMLprim value unix_utimes(value path, value atime, value mtime)
                      FILE_SHARE_READ | FILE_SHARE_WRITE,
                      NULL,
                      OPEN_EXISTING,
-                     0,
+                     FILE_FLAG_BACKUP_SEMANTICS,
                      NULL);
   caml_leave_blocking_section();
   caml_stat_free(wpath);