Add our iOS build to Flutter's customer tests

[gnprice]

This will be a followup to the pair of:

• Run iOS build in CI #773
• Add our Android build to Flutter's customer tests #774

Once we have our iOS build running in our own CI, and once we have our Android build running in Flutter's CI as well as our own, we should register our iOS build too with Flutter's "customer tests" so that they too run as part of Flutter upstream's CI.

Hopefully once those other two are done, this will be relatively straightforward, just following in the way that they paved.