Readd nightly build links of Krita to the Download page
The nightly packages are (almost) recovered, so they can be added to the downloads page again:
The new location for the nightly packages is here:
https://cdn.kde.org/ci-builds/graphics/krita/master/
Note: macOS packages are still work-in-progress, all the other packages should be available by the end of the day today, when the following pipeline finishes: