Is the certificate for that domain pinned? If not, just host it locally. In fact, I'm going to try doing that. Have you already checked the Docs? Somebody probably already published a gist somewhere with the needed path

Locally hosting platform IO is definitely on my to-do list but it is a problem which looks vast.

I have SQUID doing TLS MITM in front of it right now, but that doesn't get me really to "offline builds".