Just use a “momentary button tile” as your virtual switch. It acts exactly the way you want. Many people use them for things like representing modes or AV activities, so that the switch turns briefly on and triggers whatever you want triggered but then immediately turns itself off again so it is ready to be turned on the next time.
See the FAQ for details:
If for some reason that particular device type doesn’t work with Google home, you can use the “power allowance” option in the official SmartLights feature to have any regular binary on/off switch, including a virtual switch, turn itself off after one minute every time it is turned on.
There is a how to article in the community – created wiki on creating a virtual timer for a light which describes the set up process:
http://thingsthataresmart.wiki/index.php?title=How_to_create_a_virtual_timer_for_a_light