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Run microbenchmarks_ios only on ios/12 testbeds. (#50134)

This is to remove the flakiness associated with running those tests on
ios/13 when they always fail.

Context:
https://github.com/flutter/flutter/issues/49635
G
godofredoc committed
374b55cc608bcc555713b37afa1ce8b089c4800c
Parent: 82c00c9
Committed by GitHub <noreply@github.com> on 2/5/2020, 5:17:12 PM