ci: mkdir "$OUTPUT_DIR" -p

to solve thread 'main' (23372) panicked at src/bin/verify_rust_std/env.rs:41:5:
OUTPUT_DIR="/home/runner/work/distributed-verification/distributed-verification/assets/json" doesn't point to a valid path.

Author: zjp-CN

.github/gen_core.json.sh

@@ -5,10 +5,12 @@ set -o pipefail
 export WORKSPACE="${WORKSPACE:-$(pwd)}"
 
 export DV_LOG=off
-export OUTPUT_DIR=$WORKSPACE/assets/json
 export KANI_DIR=$WORKSPACE/kani/target/kani
 export VERIFY_RUST_STD_LIBRARY=$WORKSPACE/verify-rust-std/library
 
+export OUTPUT_DIR=$WORKSPACE/assets/json
+mkdir "$OUTPUT_DIR" -p
+
 # test/verify-rust-std needs this, so remember to
 # * update runid after verify-rust-std submodule syncs
 # * update snapshots after runid changes