haskellPackages.immortal: disable flaky tests on darwin

It took 5 tries in <https://hydra.nixos.org/eval/1809897> for the tests
to succeed on x86_64 and aarch64 darwin, respectively.

pkgs/development/haskell-modules/configuration-darwin.nix

@@ -94,6 +94,12 @@ self: super: ({
     '';
   }) super.git-annex;
 
+  # on*Finish tests rely on a threadDelay timing differential of 0.1s.
+  # You'd think that's plenty of time even though immediate rescheduling
+  # after threadDelay is not guaranteed. However, it appears that these
+  # tests are quite flaky on Darwin.
+  immortal = dontCheck super.immortal;
+
   # Prevents needing to add `security_tool` as a run-time dependency for
   # everything using x509-system to give access to the `security` executable.
   #