Fix iOS CI on Intel (for real this time) (#3232)

# Objective

- iOS CI has linker issues https://github.com/bevyengine/bevy/runs/4388921574?check_suite_focus=true

## Solution

- Building for iOS actually requires ~~both iOS SDK for target and~~ macOS SDK for build scripts. ~~I added them both when needed~~ I replaced the iOS SDK with the maOS. This was not an issue on m1 as they are compatible enough to make the build pass.
- This completely confused `shader-sys` which fails to build in this configuration. Luckily as the example now uses the new renderer, I was able to remove the old renderer and depend no more on this lib.

This is confirmed to work:
- on intel mac with simulator
- on m1 mac with simulator
- on m1 mac with real iphone

examples/ios/Cargo.toml

@@ -15,14 +15,10 @@ bevy = { path = "../../", features = [
   "bevy_audio",
   "bevy_core_pipeline",
   "bevy_gltf2",
-  "bevy_wgpu",
   "bevy_sprite2",
   "bevy_render2",
   "bevy_pbr2",
   "bevy_winit",
-  "render",
-  "png",
-  "hdr",
   "mp3",
   "x11",
   "filesystem_watcher"

examples/ios/build_rust_deps.sh

@@ -13,11 +13,12 @@ fi
 
 set -euvx
 
-# add path to the system SDK, needed since macOS 11
-if [ -z ${LIBRARY_PATH+x} ]; then
-    export LIBRARY_PATH="$(xcrun --show-sdk-path)/usr/lib"
-else
-    export LIBRARY_PATH="$LIBRARY_PATH:$(xcrun --show-sdk-path)/usr/lib"
+if [[ -n "${DEVELOPER_SDK_DIR:-}" ]]; then
+  # Assume we're in Xcode, which means we're probably cross-compiling.
+  # In this case, we need to add an extra library search path for build scripts and proc-macros,
+  # which run on the host instead of the target.
+  # (macOS Big Sur does not have linkable libraries in /usr/lib/.)
+  export LIBRARY_PATH="${DEVELOPER_SDK_DIR}/MacOSX.sdk/usr/lib:${LIBRARY_PATH:-}"
 fi
 
 # add homebrew bin path, as it's the most commonly used package manager on macOS