ocamlPackages.mirage-crypto-rng: fix tests on x86_64-darwin

pkgs/development/ocaml-modules/mirage-crypto/rng.nix

@@ -1,4 +1,6 @@
 {
+  lib,
+  stdenv,
   buildDunePackage,
   mirage-crypto,
   ohex,
@@ -24,6 +26,14 @@ buildDunePackage {
     randomconv
   ];
 
+  # test_entropy relies on timer jitter and is flaky on x86_64-darwin.
+  postPatch = lib.optionalString (stdenv.hostPlatform.isDarwin && stdenv.hostPlatform.isx86_64) ''
+    substituteInPlace tests/dune \
+      --replace-fail \
+        '(enabled_if (and (<> %{architecture} "arm64") (<> %{architecture} "riscv")))' \
+        '(enabled_if false)'
+  '';
+
   buildInputs = [ dune-configurator ];
   propagatedBuildInputs = [
     digestif