Fix a data race for NNUE

the stateInfo at the rootPos is no longer read-only, as the NNUE accumulator is part of it.
Threads can thus not share this object and need their own copy.

tested for no regression
https://tests.stockfishchess.org/tests/view/5f3022239081672066536bce
LLR: 2.96 (-2.94,2.94) {-1.50,0.50}
Total: 52800 W: 6843 L: 6802 D: 39155
Ptnml(0-2): 336, 4646, 16399, 4679, 340

closes https://github.com/official-stockfish/Stockfish/pull/2957

fixes https://github.com/official-stockfish/Stockfish/issues/2933

No functional change