issue when running gz-sim after exporting YARP_CLOCK environment variable

[LoreMoretti]

I started yarpserver and run gz sim ergocubGazevoV1_1_gz_sim_world.sdf, what I get is the following:

The gz-sim window remains black, and the following gets printed:

[INFO] |yarp.os.Port|/IITICUBLAP226/gz/62301/clock:i| Port /IITICUBLAP226/gz/62301/clock:i active at tcp://172.20.10.5:10002/
[INFO] |yarp.os.Network| Success: port-to-port persistent connection added.
[ERROR] |yarp.os.NetworkClock| Cannot find time port "/clock" or a time topic "/clock@"
[INFO] |yarp.os.Time| Waiting for clock server to start broadcasting data ...
[INFO] |yarp.os.Time| Waiting for clock server to start broadcasting data ...

with this line [INFO] |yarp.os.Time| Waiting for clock server to start broadcasting data ... being repeatedly printed on the terminal.

[traversaro]

I guess this is the same issue of robotology/gazebo-yarp-plugins#526 that was fixed for Gazebo Classic in robotology/gazebo-yarp-plugins#537, probably we can do a similar fix and a test similar to robotology/gazebo-yarp-plugins#537 ?