fix(arbtest): use --manifest-path so cargo finds workspace from /output

cargo test was running from $FUZZ_OUTPUT_DIR (/output) where there is
no Cargo.toml. Without --manifest-path, cargo fails with "could not
find Cargo.toml". Also removed unnecessary cd to /output since cargo
test doesn't write to CWD.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: randyquaye

tooling/ast_fuzzer/arbtest-container/entrypoint.sh

@@ -113,9 +113,8 @@ if [ "$FUZZ_MODE" = "reproduce" ]; then
     # Full backtrace in reproduce mode — useful for debugging the specific crash.
     export RUST_BACKTRACE=1
 
-    cd "$FUZZ_OUTPUT_DIR"
     set +e
-    cargo test --release -p noir_ast_fuzzer_fuzz "$FUZZ_TARGET" 2>&1 | tee "$FUZZ_OUTPUT_DIR/reproduce.log"
+    cargo test --release --manifest-path /noir/Cargo.toml -p noir_ast_fuzzer_fuzz "$FUZZ_TARGET" 2>&1 | tee "$FUZZ_OUTPUT_DIR/reproduce.log"
     EXIT_CODE=${PIPESTATUS[0]}
     set -e
 
@@ -143,8 +142,6 @@ export NOIR_AST_FUZZER_BUDGET_SECS="$ARBTEST_BUDGET"
 # Minimum stack size for deep AST recursion.
 export RUST_MIN_STACK="${RUST_MIN_STACK:-8388608}"
 
-cd "$FUZZ_OUTPUT_DIR"
-
 START_TIME=$(date +%s)
 ITERATION=0
 CRASH_FOUND=0
@@ -175,7 +172,7 @@ while true; do
     LOG_FILE=$(mktemp "$FUZZ_OUTPUT_DIR/iter-${ITERATION}-XXXXXX.log")
 
     set +e
-    cargo test --release -p noir_ast_fuzzer_fuzz "$FUZZ_TARGET" 2>&1 | tee "$LOG_FILE"
+    cargo test --release --manifest-path /noir/Cargo.toml -p noir_ast_fuzzer_fuzz "$FUZZ_TARGET" 2>&1 | tee "$LOG_FILE"
     EXIT_CODE=${PIPESTATUS[0]}
     set -e