Support custom cacertfile via environment variable

[rodrigues]

Hello Livebook team! Thanks for the amazing updates.

It would be nice if we could provide a custom cacertfile path for Livebook through an environment variable.

With this change, we can open notebooks from a URL inside the intranet.

I was checking Livebook.Utils.HTTP.http_ssl_opts(), castore's @cacerts is fetched at compilation time. Maybe we could put there a cacertfile: ... instead when environment variable is provided, what do you think?

Thanks! 🙏

EDIT: #2287

[josevalim]

We would move to see a PR exploring this!