The only shiny Zygarde that's ever been possible to obtain is the event one that was guaranteed shiny, with trainer name "2018 Legends". This one also comes with a special ribbon that, in addition to blocking trades over GTS or Wonder Trade, also makes sure you'll never accidentally disassemble it.
Assembling Zygarde from cells gives you chances to keep rerolling nature until you get one you like, but there will never be a chance to roll for shiny this way, as the game automatically rerolls if it would produce that in this case.
