Use hermetic toolchain on Mac, except for local iOS builds

.gn

Index: .gn
diff --git a/.gn b/.gn
index 2fe06103dfc4d1c6461bd369548c725b1d1a0f12..7591ffbb898f875c30094e6ca582ce9d1b702bc3 100644
--- a/.gn
+++ b/.gn
@@ -25,7 +25,8 @@ check_targets = [ "//webrtc/*" ]
 # These are the list of GN files that run exec_script. This whitelist exists
 # to force additional review for new uses of exec_script, which is strongly
 # discouraged except for gypi_to_gn calls.
-exec_script_whitelist = build_dotfile_settings.exec_script_whitelist
+exec_script_whitelist = build_dotfile_settings.exec_script_whitelist +
+                        [ "//build_overrides/build.gni" ]
 
 default_args = {
   # Webrtc does not support component builds because we are not using the