Rename Lambda function environment variables for AppSync API

### Is this feature request related to a new or existing Amplify category?

function, api

### Is this related to another service?

_No response_

### Describe the feature you'd like to request

The environment variables created for Lambda function when providing access to an API. Creates the following structure.

`API__GRAPHQLAPIENDPOINTOUTPUT`

Since amplify allows creating only 1 AppSync API.

### Describe the solution you'd like

Having the variable for example as `GRAPHQLAPIENDPOINTOUTPUT` would provide consistency.

### Describe alternatives you've considered

N/A

### Additional context

_No response_

### Is this something that you'd be interested in working on?

- [ ] 👋 I may be able to implement this feature request

### Would this feature include a breaking change?

- [ ] ⚠️ This feature might incur a breaking change

No files, tests, or entry points are identified. Start by locating where the Amplify CLI creates Lambda environment variables for AppSync API access, then verify the proposed variable name and assess the breaking-change implications; done means the generated variable follows the requested naming consistently.

Written by the indexing model from the issue text.