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I cannot say with certainty. There are lots of nasty critters that could be in topsoil, but for prions to be present, I would expect that the land would have had to have been grazing land, and that it had been retired from such use due to scrapie in sheep flocks, or BSE in cattle.

Documented BSE in cattle herds here is extremely rare.

In the event that the land had been used to graze sheep, and that there was scrapie known in the flock, it is possible that prions are present in the topsoil, but I would expect the construction company has removed the topsoil where any prions would be likely to be, and the blowing dust is the (generally lighter colored) mineral soil base below. The topsoil is often conserved for reapplication after landscaping when construction is complete, or may be used at another jobsite.

That, I would expect, would not be heavily contaminated (if at all), even if all other conditions had been met, simply because of the relative lack of orgaincs present (which generally accounts for the lighter color).

Note, too, that some mineral soils are deposited in environments which were swampy, and will be dark colored (usually blue-gray) as well. If there are light yellow brown or rusty colored nodules in the soil (ranging from marble sized to sometimes several feet across) usually the minerals limonite or hematite--AKA 'bog iron') and the soil is gray to blue gray in color, it usually indicates a swampy origin.

Overall, I'd figure the odds were against contamination by prions from dust blowing off the site. (Although that can be one heck of a nuisance).