fix(sendfile): Flush the out_channel used in the fallback path

Fixes ocaml#8284

Signed-off-by: Alan Hu <alanh@ccs.neu.edu>

otherlibs/stdune/src/io.ml

@@ -152,7 +152,11 @@ module Copyfile = struct
             with Unix.Unix_error (EINVAL, "sendfile", _) ->
               let ic = Unix.in_channel_of_descr src in
               let oc = Unix.out_channel_of_descr dst in
-              copy_channels ic oc)
+              copy_channels ic oc;
+              (* The stdlib's channels are buffered. After writing to oc, flush it.
+                 Note that closing oc by calling Unix.close on its underlying file
+                 descriptor does not flush oc. *)
+              flush oc)
           ~finally:(fun () ->
             Unix.close src;
             Unix.close dst)