Kovid is now mirroring the releases at GitHub as well. So if this problem happens again, you can try downloading the installer using an encrypted connection, from the releases tab on the GitHub project homepage.
Direct link to Releases:
https://github.com/kovidgoyal/calibre/releases
If you have time to kill, anyone whose ISP was doing this, can test out the new download option. Just to confirm it fixes the problem.

Kovid!