[Chess] Skip _flip in checking mate

[sotetsuk]

fn	jaxpr lines	time
_legal_action_mask	4220	944.2ms
_flip	92	27.0ms
_is_checked	206	56.2ms
_is_pseudo_legal	136	58.2ms
_hash_castling_en_passant	37	17.6ms
_apply_move	331	92.6ms
_update_history	57	27.3ms

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.64%. Comparing base (c99e282) to head (65ed174).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1230      +/-   ##
==========================================
+ Coverage   80.63%   80.64%   +0.01%     
==========================================
  Files          62       62              
  Lines        5963     5967       +4     
==========================================
+ Hits         4808     4812       +4     
  Misses       1155     1155              

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