use bash instead of shell

seems to help keep the output of the parallel scripts consistent

script/integration

@@ -58,4 +58,4 @@ fi
 
 for file in "${testfiles[@]}"; do
   echo "0$(cat .$(basename $file).time 2>/dev/null || true) $file"
-done | sort -rnk1 | awk '{ print $2 }' | xargs -I % -P $parallel -n 1 /bin/sh -c % --batch
+done | sort -rnk1 | awk '{ print $2 }' | xargs -I % -P $parallel -n 1 /bin/bash % --batch