ci: Update to actions/checkout@v4. (#1142)

This fixes annotations on task runs like the following:

    The following actions uses node12 which is deprecated and will be forced
    to run on node16: actions/checkout@v2. For more info:
    https://github.blog/changelog/2023-06-13-github-actions-all-actions-will-run-on-node16-instead-of-node12-by-default/

.github/workflows/ios.yml

@@ -12,7 +12,7 @@ jobs:
       run:
         working-directory: build
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
 
       - name: Create build directory
         run: mkdir -p build