`aws-lambda-go/lambda` appears to rewrite and break `AWS_LAMBDA_RUNTIME_API` URI scheme

**Description of issue**

When running a Go handler with the Lambda module (`github.com/aws/aws-lambda-go/lambda`), setting the env var of `AWS_LAMBDA_RUNTIME_API` to `http://localhost:2222` results in the following error:
```
2023/03/27 20:44:58 failed to get the next invoke: Get "http://http//localhost:2222/2018-06-01/runtime/invocation/next": dial tcp: lookup http: no such host
```

It appears this module does not want a URI scheme (specifically: `http://`) specified, which appears to be at odds with how other official AWS libraries that make use of the Lambda Runtime API (and `AWS_LAMBDA_RUNTIME_API` env var) seem to work.

**Expected result**

Whatever is found in the `AWS_LAMBDA_RUNTIME_API` env var is exactly the URI that should be used preceding the `/2018-06-01/runtime/invocation/next` (and other) endpoint(s).

**Steps to reproduce**

Run a module that imports `github.com/aws/aws-lambda-go/lambda` and ensure that your `AWS_LAMBDA_RUNTIME_API` starts with `http://`.

Inizia tracciando il modo in cui il modulo github.com/aws/aws-lambda-go/lambda legge AWS_LAMBDA_RUNTIME_API e costruisce l'endpoint della Lambda Runtime API. Riproduci il problema con http://localhost:2222, quindi verifica che l'URI configurata venga preservata prima di /2018-06-01/runtime/invocation/next e degli altri endpoint.

Scritto dal modello di indicizzazione a partire dal testo della issue.