nix-daemon: add variable to disable fork safety

Since macOS 10.14 this has become an error, causing problems if the
nix-daemon loads nix during substitution (this is a forked process).

Workaround for NixOS#2523.

misc/launchd/org.nixos.nix-daemon.plist.in

@@ -2,6 +2,11 @@
 <!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
 <plist version="1.0">
   <dict>
+    <key>EnvironmentVariables</key>
+    <dict>
+      <key>OBJC_DISABLE_INITIALIZE_FORK_SAFETY</key>
+      <string>YES</string>
+    </dict>
     <key>Label</key>
     <string>org.nixos.nix-daemon</string>
     <key>KeepAlive</key>