When running with --local-engine, build single-architecture apps, rather
than what's declared in the Xcode project.

By default, Flutter creates Xcode projects that generate universal
binaries supporting both armv7 and arm64 processors. Since local engine builds
are all single-architecture, override the Xcode project's ARCHS setting
with the appropriate value for the specific engine.

NOTE: this assumes engine paths consistent with those used in the engine
repo: 32-bit iOS engine builds end in _arm, 64-bit builds do not.