`build ios --config-only` doesn't handle arm64 workaround logic

(This is extremely niche so we may never fix it, but filing in case it comes up again.)

STR:

1. Check out `flutter/packages` at https://github.com/flutter/packages/pull/5915, or whatever hash that lands as.
2. `cd` into `packages/google_maps_flutter/google_maps_flutter_ios/examples/ios12`
3. `git clean -xfd .`
4. `flutter build ios --debug --no-codesign --config-only`
5. Attempt to build for a simulator from the Xcode UI

The build fails at this point. If you look at `Generated.xcconfig` it will show `EXCLUDED_ARCHS[sdk=iphonesimulator*]=i386`

Then do:
6. `flutter build ios --debug --no-codesign`

Now `Generated.xcconfig` correctly shows arm64 as an excluded architecture, and builds from the Xcode UI will work.

Expected behavior here is that `--config-only` is enough to successfully build from Xcode, with a correct `Generated.xccofig`.