do not store qsearch positions in TT as exact.

in qsearch don't store positions in TT with the exact flag.

passed STC:
https://tests.stockfishchess.org/tests/view/617f9a29af49befdeee40231
LLR: 2.95 (-2.94,2.94) <-2.25,0.25>
Total: 155568 W: 39003 L: 39022 D: 77543
Ptnml(0-2): 403, 17854, 41305, 17803, 419

passed LTC:
https://tests.stockfishchess.org/tests/view/6180d47259e71df00dcc42a5
LLR: 2.94 (-2.94,2.94) <-2.25,0.25>
Total: 79640 W: 19993 L: 19910 D: 39737
Ptnml(0-2): 37, 8356, 22957, 8427, 43

closes https://github.com/official-stockfish/Stockfish/pull/3775

Bench: 7531210