Time again to use the [kde-next] repo. Qt.57RC is already up there. Needed rebuilds are done, except Skrooge, fails so far with 5.7.
Expect Plasma 5.7Beta in kde-next soon too.
Enable by adding kde-next above build in your pacman.conf, and have them above all. Please, if you do not know how to edit your pacman.conf, then do not use kde-next, editing pacman.conf is by far the simplest part of helping with testing.