fix(store-key): Zeroize the derived MacKeys

Co-authored-by: Denis Kasak <dkasak@termina.org.uk>

crates/matrix-store-key/src/lib.rs

@@ -348,6 +348,8 @@ impl StoreKey {
     }
 }
 
+#[derive(Zeroize)]
+#[zeroize(drop)]
 struct MacKey(Box<[u8; 32]>);
 
 impl MacKey {