test_vs_head.sh: add NNUE_WEIGHTS to run an NNUE candidate vs HCE head

match_play.py's Engine passes identical engine_args to both sides, so
there was no way to get an NNUE candidate playing against the classic-
eval head_reference without patching match_play.py itself. Instead,
when NNUE_WEIGHTS is set this wraps the candidate binary in a thin
exec shim that always injects "--nnueweights $NNUE_WEIGHTS" -- works
unmodified because Engine spawns the given path directly via Popen
(no shell), so a shebang script in its place is indistinguishable
from a real binary to it. HEAD is untouched, so it keeps running HCE.

Co-Authored-By: Claude Sonnet 5 <[email protected]>

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