comment out bench/miou in dune to avoid CI build failures

since (package mirage-crypto-rng-miou-unix) is not supported without
(public_names ..) in dune, there's no easy alternative.

Marking it (optional) still results in failures with OCaml-CI

bench/dune

@@ -4,7 +4,9 @@
  (libraries mirage-crypto mirage-crypto-rng mirage-crypto-rng.unix
    mirage-crypto-pk mirage-crypto-ec))
 
-(executables
- (names miou)
- (modules miou)
- (libraries mirage-crypto-rng-miou-unix))
+; marking as "(optional)" leads to OCaml-CI failures
+; marking with "(package mirage-crypto-rng-miou-unix)" only has an effect with a "public_name"
+;(executables
+; (names miou)
+; (modules miou)
+; (libraries mirage-crypto-rng-miou-unix))