nac3standalone: work around bash mess with exit codes of substituted processes

https://unix.stackexchange.com/questions/376114/how-to-detect-an-error-using-process-substitution

nac3standalone/demo/check_demos.sh

@@ -4,7 +4,9 @@ set -e
 
 for demo in src/*.py; do
     echo "checking $demo..."
-    diff -Nau <(./interpret_demo.py $demo) <(./run_demo.sh $demo)
+    ./interpret_demo.py $demo > interpreted.log
+    ./run_demo.sh $demo > run.log
+    diff -Nau interpreted.log run.log
 done
 
 echo "PASSED"