Fix outpost calculation to use pawn attack spawns, not pawn attacks

[TheRealGioviok]

Passed STC

Test  | fixoutposts
Elo   | 4.31 +- 2.77 (95%)
SPRT  | 8.0+0.08s Threads=1 Hash=16MB
LLR   | 2.95 (-2.94, 2.94) [0.00, 3.00]
Games | N: 19582 W: 4769 L: 4526 D: 10287
Penta | [189, 2281, 4630, 2480, 211]

https://clockworkopenbench.pythonanywhere.com/test/1032/

Passed LTC

Test  | fixoutposts
Elo   | 2.42 +- 1.84 (95%)
SPRT  | 40.0+0.40s Threads=1 Hash=64MB
LLR   | 2.95 (-2.94, 2.94) [0.00, 3.00]
Games | N: 39308 W: 8867 L: 8593 D: 21848
Penta | [198, 4644, 9778, 4754, 280]

https://clockworkopenbench.pythonanywhere.com/test/1033/

Bench: 12447864