[infra] Change the iOS arch to arm64 so we get the right CIPD packages for the rpi2 machines.

Change-Id: Id44d46f57aa27b2c7e5abc9949444b244c4b7ee7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/477645
Reviewed-by: Joe Gregorio <jcgregorio@google.com>
Reviewed-by: Ravi Mistry <rmistry@google.com>
Commit-Queue: Erik Rose <erikrose@google.com>
2 files changed