[CI] iOS/tvOS 16 simulator runtime downloads intermittently fail with curl 56 timeout

## Failure signature

`Provision system dependencies` falls back to Apple's downloadable simulator index, then a legacy iOS or tvOS 16 runtime download fails after roughly 11-12 minutes with:

```text
curl: (56) Recv failure: Operation timed out
Failed to download the 16.0 simulator runtime.
System check failed
```

## Confirmed occurrences

| PR / head | Build and job | Runtime | Outcome |
|---|---|---|---|
| #26203 / `abd0ea0058d6285dccbdcc9c0fc4837d264b45d0` | [14718863 - monotouch_macos](https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=14718863&view=logs&jobId=0564bbc5-6c6b-5cb4-3d05-9ed0b2c86721) | tvOS 16.0 | Download timed out with curl 56 after about 11 minutes. Simulator-tests attempt 2 was accepted at 19:35 UTC and remains queued. |
| #26200 / `966f7cc95666adf0926c09964990868e5f161864` | [14718622 - cecil, attempt 2](https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=14718622&view=logs&jobId=803b4f8d-d709-582f-68c9-6c6e5801a94b) | iOS 16.0 | Download timed out with curl 56 after about 12 minutes. This occurred during the currently active simulator-tests retry. |

First observed: 2026-07-21 15:40 UTC. Latest observed: 2026-07-21 23:13 UTC. Recurrence: 2 jobs across 2 distinct monitored PRs.

## Classification and impact

This is a CI infrastructure/network failure, not a source or test regression. Both jobs fail while provisioning Apple simulator runtimes before tests start. The failures occur in unrelated PRs and projects, and the stable symptom is an Apple CDN transfer timing out with curl 56.

## Investigation

- Inspected both Azure timelines and the failing `Provision system dependencies` task logs.
- Confirmed that Xcode platform/component setup completed before the legacy runtime download.
- Searched open and closed `dotnet/macios` issues using the exact curl error, iOS/tvOS runtime messages, and simulator provisioning terms; no matching issue was found.

## Workaround

Rerun failed jobs after Apple CDN/network conditions recover, subject to mergeability and retry-budget gates. Do not submit another retry while an equivalent stage attempt is queued or running. Azure task logs have limited retention.

Start with the Azure timelines and the `Provision system dependencies` task logs described in the issue, confirming where the legacy iOS or tvOS 16 runtime download times out. Determine a CI-side mitigation for the Apple CDN failure, then verify that simulator provisioning completes without curl 56 timeouts across affected jobs.

Written by the indexing model from the issue text.