nix: fix installCheckPhase crashes on Darwin

With NixOS#277471 merged, nix crashes during `installCheckPhase` because
libcurl tries to call `SCDynamicStoreCopyProxies` when IPv6 is enabled.
This triggers a crash due to invoking an Objective-C `+initialize`
method after `fork` is called.

See http://sealiesoftware.com/blog/archive/2017/6/5/Objective-C_and_fork_in_macOS_1013.html.

See also NixOS/nix#2674, which addresses the issue in nix-daemon.

pkgs/tools/package-management/nix/common.nix

@@ -213,6 +213,10 @@ self = stdenv.mkDerivation {
   preInstallCheck = lib.optionalString stdenv.isDarwin ''
     export TMPDIR=$NIX_BUILD_TOP
   ''
+  # Prevent crashes in libcurl due to invoking Objective-C `initialize` after `fork`.
+  + lib.optionalString stdenv.isDarwin ''
+    export OBJC_DISABLE_INITIALIZE_FORK_SAFETY=yes
+  ''
   # See https://github.com/NixOS/nix/issues/5687
   + lib.optionalString (atLeast25 && stdenv.isDarwin) ''
     echo "exit 99" > tests/gc-non-blocking.sh