Reduce CI runtime

[uhooi]

References

• https://twitter.com/the_uhooi/status/1431637391152795650?s=20
• https://github.com/actions/cache#skipping-steps-based-on-cache-hit
• https://tech.stmn.co.jp/entry/2021/05/10/110000
• https://github.com/yonaskolb/XcodeGen/blob/master/Docs/ProjectSpec.md#sources
• https://github.com/yonaskolb/XcodeGen/issues/1006

[github-actions[bot]]

👋 Thanks for reporting! Please PR :)