dnscrypt-proxy: disable darwin build

The darwin build has been broken for a long time and I'm unable to
properly debug the issue.

What appears to be happening is that the symbol `HAVE_SANDBOX_INIT` ends
up being defined as 1 while `HAVE_SANDBOX_H` ends up being 0, resulting in
undefined reference errors when `sandbox_init()` is called (because
`<sandbox.h>` is not included first).
This is a regression from dnscrypt-proxy 1.6.0 to 1.6.1.

For context, sandbox.h is a deprecated OSX mechanism for sandboxing.

The build failure is at
https://hydra.nixos.org/build/32705849/nixlog/1/raw

This patch closes NixOS/nixpkgs#14064