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...and re-enable building arm64 isolator runtimes on regular tot bots.

The theory is that this didn't work because system Xcode on these four bots was
too old, so make it less old and try again.

Bug: 1145151
Change-Id: Ic996a91bba98e53934af8db90fdc912b8dae147b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2519206
Commit-Queue: Dirk Pranke <dpranke@google.com>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Auto-Submit: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824214}