sd-netlink: make the default timeout configurable by environment variable

sd-netlink: make the default timeout configurable by environment variable

On normal systems, triggering a timeout should be a bug in code or
configuration error, so I do not think we should extend the default
timeout. Also, we should not introduce a 'first class' configuration
option about that. But, making it configurable may be useful for cases
such that "an extremely highly utilized system (lots of OOM kills,
very high CPU utilization, etc)".

Closes #25441.