Fix audio mute on window blur

https://github.com/vector-im/element-call/pull/1015/files missed a
change of spacebarHeld to a ref.

src/useCallViewKeyboardShortcuts.ts

@@ -88,7 +88,7 @@ export function useCallViewKeyboardShortcuts(
     window,
     "blur",
     useCallback(() => {
-      if (spacebarHeld) {
+      if (spacebarHeld.current) {
         spacebarHeld.current = false;
         setMicrophoneMuted(true);
       }