Update Firestore core header path

[a-maurice]

Description

Provide details of the change, and generalize the change in the PR title above.

Update a path in the Firestore CMake to point to the internal pod path, which changed with the 10.17 dependency.

---

Testing

Describe how you've tested these changes. Link any manually triggered Integration tests or CPP binary SDK Packaging Github Action workflows, if applicable.

---

Type of Change

Place an x the applicable box:

• Bug fix. Add the issue # below if applicable.
• New feature. A non-breaking change which adds functionality.
• Other, such as a build process or documentation change.

---

Notes

• Bug fixes and feature changes require an update to the Release Notes section of release_build_files/readme.md.
• Read the contribution guidelines CONTRIBUTING.md.
• Changes to the public API require an internal API review. If you'd like to help us make Firebase APIs better, please propose your change in a feature request so that we can discuss it together.

[github-actions]

❌  Integration test FAILED

Requested by @a-maurice on commit 42b57ec
Last updated: Tue Nov 7 19:03 PST 2023
View integration test log & download artifacts

Failures	Configs
auth	[TEST] [ERROR] [Android] [1/3 os: macos] [1/2 android_device: android_target]
gma	[TEST] [FLAKINESS] [Android] [1/3 os: windows] [1/2 android_device: emulator_ftl_target] (1 failed tests)   CRASH/TIMEOUT
messaging	[TEST] [FLAKINESS] [Android] [1/3 os: macos] [1/2 android_device: android_target] (1 failed tests)   CRASH/TIMEOUT
storage	[TEST] [FLAKINESS] [iOS] [macos] [1/2 ios_device: ios_target] (1 failed tests)   FirebaseStorageTest.TestWriteAndReadFileWithCustomMetadata

Add flaky tests to go/fpl-cpp-flake-tracker