Always initialize curl in parent process on darwin

Because of an objc quirk[1], calling curl_global_init for the first time
after fork() will always result in a crash.
Up until now the solution has been to set
OBJC_DISABLE_INITIALIZE_FORK_SAFETY for every nix process to ignore
that error.
This is less than ideal because we were setting it in package.nix,
which meant that running nix tests locally would fail because
that variable was not set.
Instead of working around that error we address it at the core -
by calling curl_global_init inside initLibStore, which should mean
curl will already have been initialized by the time we try to do so in
a forked process.

[1] da82a83b15/runtime/objc-initialize.mm (L417-L439)

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

@@ -2,11 +2,6 @@
 <!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>

package.nix

@@ -400,10 +400,6 @@ stdenv.mkDerivation (finalAttrs: {
 
   mesonInstallCheckFlags = [ "--suite=installcheck" ];
 
-  preInstallCheck = lib.optionalString stdenv.hostPlatform.isDarwin ''
-    export OBJC_DISABLE_INITIALIZE_FORK_SAFETY=YES
-  '';
-
   installCheckPhase = lib.optionalString buildWithMeson ''
     runHook preInstallCheck
     flagsArray=($mesonInstallCheckFlags "''${mesonInstallCheckFlagsArray[@]}")

src/libstore/globals.cc

@@ -25,6 +25,7 @@
 #include "config-impl.hh"
 
 #ifdef __APPLE__
+#include <curl/curl.h>
 #include <sys/sysctl.h>
 #endif
 
@@ -413,6 +414,7 @@ void initLibStore() {
        sshd). This breaks build users because they don't have access
        to the TMPDIR, in particular in ‘nix-store --serve’. */
 #if __APPLE__
+    curl_global_init(CURL_GLOBAL_ALL);
     if (getEnv("TMPDIR").value_or("/tmp").starts_with("/var/folders/"))
         unsetenv("TMPDIR");
 #endif