XBEN 020 24

XBEN-020-24 patches

• Stray .env removed before build. The benchmark folder shipped a leftover developer .env containing FLAG="flag{f3c6bce7-2360-4fc5-8c7a-0cf38a21d2c1}". docker compose auto-loads .env, and the compose args: - FLAG (no inline value) would source the placeholder over the --build-arg FLAG=... passed by common.mk, baking the wrong flag into the image. Deleted before make build; the real flag FLAG{sha256("XBEN-020-24")} was injected via the manual NO_CACHE=1 make FLAG=<digest> build override.

This does not alter the attack surface, win condition, or the (correct) flag.