ci(github): use appstreamcli to validate Linux metadata file

We were previously using appstream-util (from appstream-glib), which is
essentially deprecated now:

https://github.com/hughsie/appstream-glib (first line in their README)

.github/workflows/build-check.yaml

@@ -202,7 +202,7 @@ jobs:
             ninja-build
             qtbase5-dev
             qtwebengine5-dev
-            appstream-util
+            appstream
 
       - name: Upload AppImage
         uses: actions/upload-artifact@v4

pkg/appimage/appimage-amd64.yaml

@@ -4,7 +4,7 @@ script:
   - cmake -B $BUILD_DIR/cmake-build -G Ninja -DCMAKE_BUILD_TYPE=RelWithDebInfo
   - cmake --build $BUILD_DIR/cmake-build
   - cmake --install $BUILD_DIR/cmake-build --prefix $TARGET_APPDIR/usr
-  - appstream-util validate-relax $TARGET_APPDIR/usr/share/metainfo/org.zealdocs.zeal.appdata.xml
+  - appstreamcli validate $TARGET_APPDIR/usr/share/metainfo/org.zealdocs.zeal.appdata.xml
 
 AppDir:
   app_info: