Pass Google Play credentials as environment variables

Originally mentioned in [#79](https://github.com/EFForg/apkeep/issues/79#issuecomment-1115835521):

> A prompt sounds useful, thanks. Does that code also allow sending the password
> via stdin or some env var? We're mostly using apkeep inside of automation, so a
> prompt won't help there.

Allowing users to pass credentials through environment variables would give a more secure method of authentication than a config file or command line argument.