git clone https://gitea.artixlinux.org/packages/qt6-base.git
cd qt6-base
In the PKGBUILD change line 83 to:
source=(git+https://code.qt.io/qt/$_pkgfn#tag=v$pkgver-beta3
Change line 87 to
sha256sums=('b9200a629a12917b33748a6cb25880e9429e294b03623011201a7195a888f36f'
or
sha256sums=('SKIP'
To build and install (add r to the options if you don't want to keep any 'makedepends' installed):
makepkg -si