Better nightly builds on CI, state of Qt 6 support, and steps towards v3

Today I've worked on improving the nightly build support on CI, and nightly builds are now finally available as individual files instead of being zipped into one big one.

This means it's now easier for people on macOS and Windows to test the latest changes before they are released, by simply going to the latest nightly build and grabbing a macOS/Windows build under "Artifacts".

This now also includes builds of the qt6-v2 branch, and I'd really welcome having some more testers there! qutebrowser developers (and some brave[*] users) have been using it as daily driver since a while, and it's generally regarded to be ready to use. If you are on Linux, there's support, Flatpak builds and an Archlinux AUR package available too, see the issue for all the details.

Please give it a spin and open a new issue if you run into any trouble!

There still is a lot of work to do to actually integrate this into the master branch and getting qutebrowser v3.0.0 out, but most of the remaining work is stuff behind the scenes now - from an user's perspective, the qt6-v2 branch should be pretty much ready.

For the next few weeks, my plan is to get back to merging some PRs, and regularily rebase the qt6-v2 branch on master to get them in there as well. I'll also start teaching Python at the university again in September (until February), so the clock is ticking a bit... I'm hoping that I'll still be able to work on qutebrowser and get this all wrapped up on the side though (only teaching two days a week, plus preparation/admin stuff).

[*] The English word, not the browser ;)