[code-infra] Fix nextjs build cache

[JCQuintas]

Main changes:

• Cache only docs/.next/cache folder instead of the entire docs/.next folder
  • We were previously caching some files that could get stale. This is in-line with the official plugin
    https://github.com/netlify/next-runtime/blob/733a0219e5413aa1eea790af48c745322dbce917/src/build/cache.ts
• Remove digest
  • providing a digest could make it worse, since it was meant for node_modules-like folders, which contain an ungodly amount of files
    docs: https://github.com/netlify/build/blob/main/packages/cache-utils/README.md#digests
    save/restore impl: https://github.com/netlify/build/blob/main/packages/cache-utils/src/main.ts
    manifest impl: https://github.com/netlify/build/blob/main/packages/cache-utils/src/manifest.ts
• Move from onPostBuild to onBuild to be in par with the official next plugin.
  • Not sure this is useful, but leaves us with a behaviour similar to the official plugin 🤷
    https://github.com/netlify/next-runtime/blob/733a0219e5413aa1eea790af48c745322dbce917/src/index.ts

Note: Should I also update the version in package.json? 🤔

[mui-bot]

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[LukasTy]

The change makes sense to me. 👍
Thank you for handling it. 🙏
Let's test it and see if we can avoid the ocassional build failures. 🤞

Should I also update the version in package.json? 🤔

I think that it won't hurt bumping the version thus aligning it with @mui/material. 👍

[Janpot]

👍 Yes, removing the digests make sense. Still not 100% clear to me how this can cause the issue though. Next.js already only retains the .next/cache folder during a build. It still feels to me like there's something going wrong around the github: dependency.