Update CodeSandbox to pull build artifacts from CI

[acdlite]

Instead of building them from source.

The download-experimental-build script polls CI until the build has finished.

[rickhanlonii]

Were we doing the experimental build before, or is that new?

[sizebot]

Comparing: 51e017c...92f42ad

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.min.js	=	129.52 kB	129.52 kB	=	41.28 kB	41.28 kB
oss-experimental/react-dom/cjs/react-dom.production.min.js	=	132.34 kB	132.34 kB	=	42.22 kB	42.22 kB
facebook-www/ReactDOM-prod.classic.js	=	411.10 kB	411.10 kB	=	76.08 kB	76.08 kB
facebook-www/ReactDOM-prod.modern.js	=	399.67 kB	399.67 kB	=	74.37 kB	74.37 kB
facebook-www/ReactDOMForked-prod.classic.js	=	411.10 kB	411.10 kB	=	76.08 kB	76.09 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

Generated by 🚫 dangerJS against 92f42ad

[acdlite]

@rickhanlonii I think it was using the stable ones but not intentionally, just because that's the default for the build command

[rickhanlonii]

Should it be stable/alpha until 18 lands?

[acdlite]

@rickhanlonii The reason I went with experimental is because it's a superset of the features in stable.

I think the best argument for keeping them as stable is if someone submits a bug report that only affects stable, then the CodeSandbox on a bugfix PR could show it as "fixed" even if it's still broken in stable. But since our bugfix PRs are required to include a jest regression test anyway I'm not too worried about that.