You would have to implement this configuration and logic. It would have to evaluate if the build has been triggered and running and then wait for it to finish.
Maybe something can be done in order to check with the buildSteps and a waitFor or with a custom builder.
A combination between a call to the API and listing the Cloud Builds and they're statuses, implementing maybe waitFor in your config so it checks and then proceed with the build once the other is finished.
The thing is depending on how many builds you are submitting I don't have a clear idea on how you can assign a priority in the queue as it would go FIFO.
Someone else added a similar question here
Hope this helps.