Update build workflow to use the C++ version input

[AlmostMatt]

Description

The C++ version input was being ignored and empty string was set instead.

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Testing

Ran this workflow and verified that the Print Outputs step printed outputs.firebase_cpp_sdk_version as a non-empty string
https://github.com/firebase/firebase-unity-sdk/actions/runs/3115233731/jobs/5051915301

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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.

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[github-actions]

❌  Integration test FAILED

Requested by @firebase-workflow-trigger[bot] on commit 9729a02
Last updated: Fri Sep 23 21:33 PDT 2022
View integration test log & download artifacts

Failures	Configs
analytics	[TEST] [ERROR] [2019] [macos] [1/6 Platform(s): iOS] [1/3 Test Device(s): simulator_target]
auth	[TEST] [ERROR] [2019] [macos] [1/6 Platform(s): iOS] [1/3 Test Device(s): ios_target]
firestore	[BUILD] [ERROR] [2019] [macos] [1/3 Platform(s): iOS] [TEST] [FAILURE] [2019] [macos] [1/6 Platform(s): windows]
functions	[TEST] [ERROR] [2019] [macos] [1/6 Platform(s): iOS] [1/3 Test Device(s): simulator_target]
installations	[TEST] [ERROR] [2019] [macos] [1/6 Platform(s): iOS] [1/3 Test Device(s): simulator_target]
messaging	[TEST] [ERROR] [2019] [macos] [1/6 Platform(s): iOS] [1/3 Test Device(s): ios_target] [TEST] [FAILURE] [2019] [macos] [1/6 Platform(s): iOS] [1/3 Test Device(s): simulator_target]
remote_config	[TEST] [ERROR] [2019] [macos] [1/6 Platform(s): iOS] [1/3 Test Device(s): simulator_target]
storage	[TEST] [FAILURE] [2019] [macos] [2/6 Platform(s): ubuntu windows]