Cache and expose serialized build values

[berekuk]

Right now we store only build errors; could be useful to store build outputs too (probably in S3-like object storage, i.e. DigitalOcean Spaces).

Important: this requires figuring out permissions; e.g. it's possible to import a private model from a public model, and someone with access only to the public model shouldn't see build outputs, or even build errors.

[berekuk]

Important: https://github.com/quantified-uncertainty/squiggle/issues/3497#issuecomment-2583432040