Load :prezto:module:ssh:load identities on macOS

If the user defines identities in `:prezto:module:ssh:load`, always try
to load them, even on macOS.  On macOS, also try to load Keychain
managed identities. (Assume if a user adds an identity to Keychain then
the use would want those identities automatically loaded.)

modules/ssh/init.zsh

@@ -44,12 +44,16 @@ if ssh-add -l 2>&1 | grep -q 'The agent has no identities'; then
   # passphrase.
   if [[ -n "$DISPLAY" && -x "$SSH_ASKPASS" ]]; then
     ssh-add "${_ssh_identities:+$_ssh_dir/${^_ssh_identities[@]}}" < /dev/null 2> /dev/null
-  elif [[ "$OSTYPE" == darwin* ]]; then
-    # macOS: `ssh-add -A` will load all identities defined in Keychain
-    ssh-add -A 2> /dev/null
   else
     ssh-add "${_ssh_identities:+$_ssh_dir/${^_ssh_identities[@]}}" 2> /dev/null
   fi
+
+  if [[ "$OSTYPE" == darwin* ]]; then
+    # macOS: `ssh-add -A` will load all identities defined in Keychain.
+    # Assume `/usr/bin/ssh-add` is Apple customized version that understands
+    # the `-A` switch.
+    /usr/bin/ssh-add -A
+  fi
 fi
 
 # Clean up.