[build] Don't attempt to use hermetic toolchain on iOS

iOS still uses the recipe-side toolchain, so calling the mac_toolchain
script on src-side is useless in iOS checkouts.

Bug: chromium:986701
Change-Id: I1252d01088eca69e6b465c30856f9434eea99aab
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1725612Reviewed-by: Tamer Tas <tmrts@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62989}

DEPS

@@ -414,7 +414,7 @@ hooks = [
     # Update the Mac toolchain if necessary.
     'name': 'mac_toolchain',
     'pattern': '.',
-    'condition': 'checkout_ios or checkout_mac',
+    'condition': 'checkout_mac',
     'action': ['python', 'v8/build/mac_toolchain.py'],
   },
   # Pull binutils for linux, enabled debug fission for faster linking /