Turn on build script on Kokoro.

[SortaCookie]

Bug: 495549526

[github-actions]

🤖 Gemini Suggested Commit Message

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build: Enable Kokoro tvOS arm64 build script

The build_mac.sh script is now activated for tvOS arm64 builds
on Kokoro. This change enables the previously commented-out
build process, resolving the corresponding TODO.

Bug: 495549526

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[gemini-code-assist]

Code Review

This pull request updates the tvOS arm64 build configuration by enabling the 'build_mac.sh' script and removing the corresponding TODO. It also bumps the subproject commit versions for 'harfbuzz-ng' and 'pdfium'. I have no feedback to provide as there were no review comments.