Set apple.yml test-infra checkout to main instead of empty ref

The mobile_job call passed `test-infra-ref: ''`, which makes
actions/checkout resolve the default branch via a REST call and, under CI
fan-out, contributes to GitHub App installation rate-limit exhaustion
(meta-pytorch/pytorch-gha-infra#1375). An explicit ref skips that lookup;
`main` is the branch the empty ref already resolved to.

This is the one call site that overrode pytorch/test-infra#8472's new
`main` default with an explicit empty string.

Author: huydhn

.github/workflows/apple.yml

@@ -161,7 +161,7 @@ jobs:
       device-type: ios
       # For iOS testing, the runner just needs to call AWS Device Farm, so there is no need to run this on macOS
       runner: linux.2xlarge
-      test-infra-ref: ''
+      test-infra-ref: main
       # This is the ARN of ExecuTorch project on AWS
       project-arn: arn:aws:devicefarm:us-west-2:308535385114:project:02a2cf0f-6d9b-45ee-ba1a-a086587469e6
       # This is the custom device pool that only includes iOS devices