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

[stuartmorgan-g]

(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 [google_maps_flutter] Restore the arm64 workaround packages#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.

[jmagman]

updateGeneratedXcodeProperties is running before pod install.

flutter/packages/flutter_tools/lib/src/ios/mac.dart

Lines 242 to 247 in a3c11cd

	await updateGeneratedXcodeProperties(
	project: project,
	targetOverride: targetOverride,
	buildInfo: buildInfo,
	);
	await processPodsIfNeeded(project.ios, getIosBuildDirectory(), buildInfo.mode);

However the generated EXCLUDED_ARCHS logic relies on pod install having been run and generating the Pods.xcodeproj

flutter/packages/flutter_tools/lib/src/xcode_project.dart

Lines 193 to 199 in a3c11cd

	final Directory podXcodeProject = hostAppRoot
	.childDirectory('Pods')
	.childDirectory('Pods.xcodeproj');
	if (!podXcodeProject.existsSync()) {
	// No plugins.
	return true;
	}

(so how does this work without --config-only)

[jmagman]

@gaaclarke also reported this #143133