Make widgets take up the full height of the PiP again (#10886)

This fixes a visual regression introduced in 73007d6dd6: AppTiles with the class mx_AppTile_mini don't get an mx_AppsDrawer as their parent, so the --AppTile_mini-height variable needs a broader scope.

res/css/views/rooms/_AppsDrawer.pcss

@@ -15,8 +15,11 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-.mx_AppsDrawer {
+:root {
     --AppTile_mini-height: 220px;
+}
+
+.mx_AppsDrawer {
     --minWidth: 240px; /* TODO this should be 300px but that's too large */
 
     margin: var(--container-gap-width);