Use xbuild instead of cargo apk in the CI

In addition to cargo apk being deprecated, xbuild also has support for
ios.

Signed-off-by: John Nunley <dev@notgull.net>

xbuild-target/src/main.rs

@@ -0,0 +1,6 @@
+fn main() {
+    winit::event_loop::EventLoop::new()
+        .unwrap()
+        .run(|_, _, _| todo!())
+        .ok();
+}