Fix EIO build errors

[georgyo]

Fixes the following build errors

Entering directory '/home/shammas/tmp/charrua'
File "vendor/rawlink/lib/eio_rawlink.ml", line 42, characters 12-25:
42 |     let n = Eio.Flow.read t.flow t.buffer in
                 ^^^^^^^^^^^^^
Error (alert deprecated): Eio.Flow.read
Use single_read if you really want this.
File "vendor/rawlink/lib/eio_rawlink.ml", line 21, characters 2-23:
21 |   fd : Unix.file_descr;
       ^^^^^^^^^^^^^^^^^^^^^
Error (warning 69 [unused-field]): record field fd is never read.
(However, this field is used to build or mutate values.)