Use https apt sources where possible

[ids1024]

Modern versions of apt have https support builtin, but repositories are typically used over http. Historically ssl has been considered unnecessary given gpg signature checking, but it's probably better to use it where possible these days.

• https://apt.pop-os.org/proprietary/ works over https
• https://us.archive.ubuntu.com/ubuntu/ fails to connect
• https://ppa.launchpad.net/system76/pop/ubuntu connects over https, but with a certificate for the wrong domain, and fails to serve the required files with certificate checking disabled

So it looks like we should be able to use https sources for what we host ourselves, I guess.