sd-id128: look for invocation id in environment first, keyring second

sd-id128: look for invocation id in environment first, keyring second

As general principle, we generally check command line args first, the
enviroment second, and external configuration and system state only later.
In case of the invocation ID, checking the keyring before the environment
was implemented as a poor-man's security measure. But this is not really
useful, since we're moving within the same security boundary. So let's just
do the expected thing, and check environment first.

Prompted by https://github.com/systemd/systemd/pull/11991#issuecomment-474647652.