Bundling the source tree in the installer is GPLv3 § 6(a) -- when providing a physical medium.
As long as calibre's GitHub repo is available, you can direct people there, I believe.
But you are responsible for hosting it yourself in the event calibre becomes abandoned and is no longer available from GitHub or the official website.
"Hosting yourself" -- includes a fork on GitHub.

You can host the tarball yourself just in case GitHub goes out of business, which, while also pretty unlikely, would put you in a spot of bother.
Disclaimer: IANAL, but this is how I understand it.