Add iOS and macOS generated files to CI

[gnprice]

Following up on #319 (and #60), it'd be good to have our CI check that we've remembered to update the handful of files in ios/ and macos/ that regularly need an update when we add or upgrade a dependency, notably the Podfile.lock files.

I think this is just a matter of running the following two commands:

flutter build ios --config-only
flutter build macos --config-only

and checking they didn't change anything, in the same way we do for several other suites in tools/check.

The one wrinkle is that such a check will need to run on macOS. GitHub does offer macOS-based runners for actions:
https://docs.github.com/en/actions/using-github-hosted-runners/about-github-hosted-runners/about-github-hosted-runners#supported-runners-and-hardware-resources

There's a steep 10x multiplier in how those count against our quota of free usage (and a similar steeper rate if paying for more):
https://docs.github.com/en/billing/managing-billing-for-github-actions/about-billing-for-github-actions#minute-multipliers
so we'll want to stick to Linux for all the suites where we can. But that shouldn't stop us from using macOS where necessary.

We can also arrange for that CI job to only run when a relevant file was touched. That will be a small fraction of PRs, so should substantially cut down on any cost; and conversely, this is an area where if anything does slip through, it's impossible for it to affect the app's behavior (because the files will get automatically updated when building), so the impact is only a dev annoyance and it's fine to risk that and see if it happens.