Depends on what it was. tee hee
If it was something I could scoop up with a handful or two (chips or something) then I would at least attempt to get the bulk of it. If it was something rolly, like skittles or cheerios that just rolled to kingdom come, then I would probably just use my foot to scoot it out of the aisle so it didn't hang up on the wheels of the carts.
It it was wet/sticky, then I'd look for a store employee.
I generally never allowed my kids too many snacks that were that messy. They may have gotten a small ziploc bag, but they never dropped it and had things go everywhere. A cheerio or two, yeah, but never an entire BAG.