nix: fix installCheckPhase crashes on Darwin

Ensure that `OBJC_DISABLE_INITIALIZE_FORK_SAFETY=yes` is set when
starting the Nix daemon during tests, or the fetchurl.sh test will
crash trying to initialize libcurl. This is happening since IPv6 support
was enabled in https://github.com/NixOS/nixpkgs/pull/277471.

See also:
- http://sealiesoftware.com/blog/archive/2017/6/5/Objective-C_and_fork_in_macOS_1013.html
- https://github.com/NixOS/nix/pull/2674

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

@@ -213,6 +213,11 @@ self = stdenv.mkDerivation {
   preInstallCheck = lib.optionalString stdenv.isDarwin ''
     export TMPDIR=$NIX_BUILD_TOP
   ''
+  # Prevent crashes in libcurl due to invoking Objective-C `+initialize` methods after `fork`.
+  # See http://sealiesoftware.com/blog/archive/2017/6/5/Objective-C_and_fork_in_macOS_1013.html.
+  + 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