Move scanbuild CI stage back to flatpak and rebuild CI image
In !953 (merged) I temporarily switched our scan-build stage from using the GNOME flatpak runtime to using Fedora 34. We should switch back to the GNOME runtime when https://github.com/mesonbuild/meson/issues/8707 is fixed.
Also, the v2 CI image is bloated with Epiphany build dependencies and clang-analyzer. After switching the scan-build job back to using the GNOME runtime, we could shrink the image size by >5x by using a v3 image with these deps removed.