AWS credentials using credential_process

## Description

If `~/.aws/credentials` uses [credential_process](https://docs.aws.amazon.com/cli/latest/userguide/cli-configure-sourcing-external.html), `up` will fail with a "Cannot find credentials" error.

## Steps to Reproduce

`up.json`

```json
{
"profile": "developer"
}
```

`~/.aws/credentials`

```
[developer]
credential_process = /opt/bin/awscreds-custom --username helen
```

Try to deploy:
```sh
❯ up

build: 5 files, 12 MB (545ms)

Error: Cannot find credentials, visit https://apex.sh/docs/up/credentials/ for help.
```

## Workaround

Setting the environment variable `AWS_SDK_LOAD_CONFIG` will instruct the underlying AWS SDK to use [Shared Config Fields](https://docs.aws.amazon.com/sdk-for-go/api/aws/session/#hdr-Sessions_from_Shared_Config) (thus executing the custom `credential_process`).

```sh
AWS_SDK_LOAD_CONFIG=1 up
```

---
I'm not sure there's much you could do in your AWS session implementation beyond force-enable Shared Config support. Opening this as mostly as a reference for anyone who comes across the error with this configuration.