Whatever it's flushed with needs to be volatile so that it evaporates completely. Unleaded is good, turps not so good, pure alcohol is okay but too tempted to waste it cleaning an intercooler.
If you flush it with unleaded don't worry too much about fumes. You can mix very small amounts of unleaded into diesel without harm - unleaded fuel will change the nature of the combustion properties of diesel, but in extremely small amounts like the residual vapour in the intercooler, you wouldn't notice any effect.