Add nightly CI build against sentry-cocoa `main`

Today our podspec pins `s.dependency 'Sentry', '9.12.1'`. We discover regressions or API changes in sentry-cocoa only when:

1. A user reports a broken build after upgrading sentry-cocoa, or
2. We manually bump the pin during a release cycle.

This is too late, especially as sentry-cocoa progresses on:

* getsentry/sentry-cocoa#7598 — the new `SentryObjC` wrapper
* [getsentry/sentry-cocoa#7881]() — the public-API expansions we requested

Any of these may rename, remove, or change behavior of APIs we depend on. We want to know before users do. A nightly GitHub Actions job builds the RN sample app against the latest sentry-cocoa `main` and reports failures.