Is cheese an adjective in the word cheese pizza'?
Nouns often do the work of adjectives in phrases of this kind. "Boat launch", "Refrigerator Truck", "Vacuum Cleaner Salesman", "Toilet Brush", "Taco Stand", "Moose Hide", "Dog Collar" are all similar examples: boat, refrigerator, vacuum cleaner, toilet, taco, moose and dog are all nouns. But each one of them modifies the following noun, and so does the work of an adjective.
This kind of modification is quite common in languages like Chinese, and really saves a lot of time you would otherwise have to spend talking about a "pizza which has cheese and no other toppings", a "stand which primarily sells tacos", a "salesman who sells vacuum cleaners" or a "collar which is intended for use by dogs."