ci: add qt6-shadertools, gsettings-qt, and fail-fast=false for Arch Linux Qt6 build

- Add qt6-shadertools (required for Qt6 shader compilation)
- Add gsettings-qt to Qt6 dependencies (was missing, causing build issues)
- Set fail-fast=false so both Qt5 and Qt6 jobs run independently

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: zccrs

.github/workflows/dtkdeclarative-archlinux-build.yml

@@ -10,6 +10,7 @@ jobs:
     runs-on: ubuntu-latest
     container: archlinux:latest
     strategy:
+      fail-fast: false
       matrix:
         include:
           - qt_version: 5
@@ -44,13 +45,13 @@ jobs:
         if: matrix.qt_version == 6
         run: |
           pacman -S --noconfirm --noprogressbar \
-            qt6-base qt6-tools qt6-declarative \
+            qt6-base qt6-tools qt6-declarative qt6-shadertools \
             qt6-wayland qt6-svg qt6-imageformats \
             extra-cmake-modules \
             gtest \
             librsvg libraw \
             libqtxdg \
-            icu uchardet dbus spdlog \
+            icu uchardet dbus spdlog gsettings-qt \
             dtkcommon dtk6log \
             treeland-protocols dtk6core dtk6gui