fix(core, ios): ensure iOS SDK version can be found from Package.swift

[russellwheatley]

Description

• Increased swift integration test timeout which was the reason why it was failing. It also matches the timeout set in Firestore PR: https://github.com/firebase/flutterfire/pull/13329/files#diff-61a085d0578afbe6a13da0c8926054ea59e83ce30bd655a6349e78afe7940351R127

• I was able to reproduce the bug on a throwaway Flutter app outside of Flutterfire. I tested by pointing to firebase_core on path with the fix and I was able to build successfully:

Related Issues

fixes #13726

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Breaking Change

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[kuhnroyal]

Tested this locally with Git dependency, no more build errors. Thanks!