ci: switch back to Qt 5 for Windows, requires installer adjustment.

.github/workflows/build.yml

@@ -104,9 +104,16 @@ jobs:
     - name: Checkout LFS objects
       run: git lfs checkout
 
-    # Windows/macOS only:
-    - name: Install Qt
-      if: startswith(matrix.config.os, 'windows') || startswith(matrix.config.os, 'macos')
+    # Windows only:
+    - name: Install Qt 5
+      if: startswith(matrix.config.os, 'windows')
+      uses: jurplel/install-qt-action@v4
+      with:
+        cache: true
+
+    # macOS only:
+    - name: Install Qt 6.5
+      if: startswith(matrix.config.os, 'macos')
       uses: jurplel/install-qt-action@v4
       with:
         cache: true