How do foundries prevent zinc from boiling away when alloyed with Aluminum?
When alloys are made by mixing molten metals (actually an alloy only need contain one metal and at least one other compound, metal or not) the metals only need to be heated to their melting point, not all the way to their boiling point. In the example you've given, the melting point of aluminum is $\pu{660^oC}$, which is $\pu{247^oC}$ below the boiling point of zinc, so the volatilization of zinc is negligible under these conditions.  

However, the issue you bring up does present problems in other cases. For example this article states the following:  


  One difficulty in making alloys is that metals have different melting points. Thus copper melts at $\pu{1,083^oC}$, while zinc melts at $\pu{419^oC}$ and boils at $\pu{907^oC}$. So, in making brass, if we just put pieces of copper and zinc in a crucible and heated them above $\pu{1,083^oC}$, both the metals would certainly melt. But at that high temperature the liquid zinc would also boil away and the vapour would oxidize in the air. The method adopted in this case is to heat first the metal having the higher melting point, namely the copper. When this is molten, the solid zinc is added and is quickly dissolved in the liquid copper before very much zinc has boiled away. Even so, in the making of brass, allowance has to be made for unavoidable zinc loss which amounts to about one part in twenty of the zinc. Consequently, in weighing out the metals previous to alloying, an extra quantity of zinc has to be added.  


Summary, TL;DR:
In your example of aluminum and zinc, each metal melts well below either of their boiling points so that loss via volatilization is not a problem. There are cases however, such as alloying copper and zinc, where the boiling point of one metal is lower than the melting point of the other. One way to minimize (but not eliminate) the loss of the more volatile metal is to quickly dissolve it in the high-melting metal and then cool the solution. Although this does not eliminate losses due to volatilization, it can greatly reduce the problem. And actually, since alloys are frequently composed of predominantly one metal, it is not uncommon to dissolve the lesser components into the primary component as a matter of practice anyway.  

I hope the example I gave addresses your question. Please don't hesitate to ask for any clarifications in the comments below.