Fix output file for previous artifacts

circle.yml

diff --git a/circle.yml b/circle.yml
index bb716dd..b6ae60d 100644
--- a/circle.yml
+++ b/circle.yml
@@ -145,7 +145,7 @@
       curl $(curl https://circleci.com/api/v1/project/mathquill/mathquill/latest/artifacts \
               | json -a url pretty_path -d '\n\t' \
               | grep '\.png$' \
-              | sed 's:\$CIRCLE_ARTIFACTS/imgs/:-o /tmp/PREV_:')
+              | sed "s:\$CIRCLE_ARTIFACTS/imgs/:-o $CIRCLE_ARTIFACTS/imgs/PREV_:")
 
     # finally, complain to Circle CI if there were nonzero test failures
     - |-