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

This is a no-op on iOS, since there the system or recipe side sdk is
used.

Bug: chromium:986701
Change-Id: I1daffa748b0dadd074a9c8ca2b756416c197ca1b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1725914Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682422}

DEPS

@@ -2388,7 +2388,7 @@ hooks = [
     # Update the Mac toolchain if necessary.
     'name': 'mac_toolchain',
     'pattern': '.',
-    'condition': 'checkout_ios or checkout_mac',
+    'condition': 'checkout_mac',
     'action': ['python', 'src/build/mac_toolchain.py'],
   },
   # Pull binutils for linux, enabled debug fission for faster linking /