pub fn with_prefix(prefix: &str) -> Self {
        EnvironmentProvider {
            prefix: prefix.to_owned(),
        }
    }
Create an EnvironmentProvider with a non-standard variable prefix.

```rust
use std::future::Future;
use rusoto_credential::{EnvironmentProvider, ProvideAwsCredentials};
use std::env;

#[tokio::main]
async fn main() -> () {
env::set_var("MYAPP_ACCESS_KEY_ID", "ANTN35UAENTS5UIAEATD");
env::set_var("MYAPP_SECRET_ACCESS_KEY", "TtnuieannGt2rGuie2t8Tt7urarg5nauedRndrur");
env::set_var("MYAPP_SESSION_TOKEN", "DfnGs8Td4rT8r4srxAg6Td4rT8r4srxAg6GtkTir");

let creds = EnvironmentProvider::with_prefix("MYAPP").credentials().await.unwrap();

assert_eq!(creds.aws_access_key_id(), "ANTN35UAENTS5UIAEATD");
assert_eq!(creds.aws_secret_access_key(), "TtnuieannGt2rGuie2t8Tt7urarg5nauedRndrur");
assert_eq!(creds.token(), &Some("DfnGs8Td4rT8r4srxAg6Td4rT8r4srxAg6GtkTir".to_string()));
assert!(creds.expires_at().is_none()); // doesn't expire

env::set_var("MYAPP_CREDENTIAL_EXPIRATION", "2018-04-21T01:13:02Z");
let creds = EnvironmentProvider::with_prefix("MYAPP").credentials().await.unwrap();
assert_eq!(creds.expires_at().unwrap().to_rfc3339(), "2018-04-21T01:13:02+00:00");
}
```
rusoto/credential/src/environment.rs