_BUILD_TIMEOUT not respected in AWS Amplify — build still cancels after 30 minutes

### Environment information

```plain text
Service: AWS Amplify Hosting

Build system: Amplify Console (CodeBuild backend)

Build configuration: Using amplify.yml

Runtime: Node.js 20

Region: (add your region, e.g. eu-west-2)

Branch setup: Multiple branch environments, each with _BUILD_TIMEOUT defined in Amplify environment variables
```

### Describe the bug

Even after defining `_BUILD_TIMEOUT` as an environment variable (both in **Amplify App Environment Variables** and **Branch-specific Environment Variables**), Amplify deployments are still being **cancelled automatically after ~30 minutes**, which matches the default build timeout.

It appears that the `_BUILD_TIMEOUT` environment variable is **ignored** by Amplify’s underlying CodeBuild configuration.

### Reproduction steps

### 🔁 **Reproduction Steps**

1. Go to Amplify Console → App → **App Settings → Environment Variables**.
2. Add the variable:

```
_BUILD_TIMEOUT=90
```
3. Also add the same variable under **Branch settings → Environment variables**.
4. Trigger a new deployment (build takes >30 min).
5. Observe that Amplify **terminates the build after 30 minutes** despite `_BUILD_TIMEOUT` being set.

---

### ⚙️ **Expected Behavior**

Amplify should extend the build duration based on `_BUILD_TIMEOUT` — or provide a documented, working method to configure build timeout per branch/environment.

```

---

### 💬 **Additional Context**

There is no clear documentation on whether `_BUILD_TIMEOUT` is a supported environment variable in Amplify Hosting or only in Amplify CI/CD pipelines.
If not supported, there should be a documented way to increase build time beyond 30 minutes without switching to manual CodeBuild project control.

Start with the amplify.yml configuration and AWS Amplify Hosting documentation for supported build-time environment variables and timeout settings. Reproduce the deployment with _BUILD_TIMEOUT=90 in both app and branch environment variables, then confirm whether the build still stops at 30 minutes. Done means identifying a supported configuration or documenting that the variable is not supported and explaining the available alternative.

Written by the indexing model from the issue text.