fix: Address flaky dispatch

[ncooke3]

Goal: When nightly testing, disable a dispatch_async to observe whether that makes a difference.

Context: I saw promising results in reducing test flakiness when making removing an async dispatch in CLSSettings. Before updating the tests or SDK to address this more robustly, I'd like to test disabling the async dispatch for a few nights to see if this actually makes a difference.

#no-changelog

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[ncooke3]

/gemini review

[gemini-code-assist]

Code Review

This pull request aims to address test flakiness by disabling an asynchronous dispatch during nightly tests. This is achieved by introducing a FIREBASE_IS_NIGHTLY_TESTING preprocessor macro. The changes in Package.swift correctly set up this macro based on an environment variable and refactor the FirebaseCrashlytics target definition into a helper function for better organization. The change in FIRCLSSettings.m uses this macro to conditionally compile out the dispatch_async call. My feedback focuses on improving the readability of the conditional compilation in FIRCLSSettings.m. Overall, the changes are logical and achieve the stated goal.

[ncooke3]

cc: @themiswang

[ncooke3]

So far, no flakes from this test file in the SPM nightly workflows: https://github.com/firebase/firebase-ios-sdk/actions/runs/20258318035

Will give it a few more days before making fix more robust.