fix: updated env var in real time

[Josephasafg]

There was a bug where if you were to set an env var after AI21Client was imported, to not receive the updated value of the variable

[Josephasafg]

That was the use case -

import os
from ai21 import AI21Client

os.enviorn["AI21_API_KEY] = "my-key"

client = AI21Client()

This implies that not setting api_key to anything is okay, and under the hood the code will check os.environ["AI21_API_KEY"]. But that doesn't seem to be true. Simply instantiating the client as client=AI21Client() fails to run with a missing API key, even though in the previous line I wrote:
os.environ["AI21_API_KEY"] = "...my key here..."

This was caused due to not fetching the env var in real time but only on first import of the client