Fix iOS simulator runtime unavailable error on Intel Macs

On Intel (x86) Macs, the iOS simulator runtime may be cached as an
ARM-only version. When this happens, xcrun simctl list runtimes reports
the runtime as unavailable with the error:
(unavailable, liblaunch_sim.dylib could not be opened)

Added _unavailable_runtime() method to detect this architecture mismatch
error. The __call__ method now checks for both missing runtimes AND
unavailable runtimes. When either condition is detected, it sets
_cleanup_cache = True and retries _setup_osx_sdk to wipe the cache and
re-download the correct universal runtime.

Change-Id: Ib1d3d09907be1d591c42a0ca1e946b0bdfda2557
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/74140
Reviewed-by: Victoria Ashworth <vashworth@google.com>
Commit-Queue: Victoria Ashworth <vashworth@google.com>