For Windows packaging, attempt to speed things up with a pre-merge step.

[jonsimantov]

This parallelizes the symbol renaming, with the final merge coming at the end.

Description

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Testing

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

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Type of Change

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• [ ] 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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Notes

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• 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.