CI: Ignore diffs when fetching baseline screenshots

Also when ignoreing pieces and baselines, fixed string not regex

circle.yml

diff --git a/circle.yml b/circle.yml
index 67e81cb..be4994b 100644
--- a/circle.yml
+++ b/circle.yml
@@ -195,8 +195,9 @@
       baseline_imgs="$(echo "$artifacts_json" \
                     | $json -a url pretty_path -d '\n\t' \
                     | grep '\.png$' \
-                    | grep -v '/pieces/' \
-                    | grep -v '/baseline/' \
+                    | grep -v '_DIFF\.png$' \
+                    | grep -vF '/pieces/' \
+                    | grep -vF '/baseline/' \
                     | sed "s:\$CIRCLE_ARTIFACTS/imgs/:-o $CIRCLE_ARTIFACTS/imgs/baseline/:")"
       echo 'Baseline image URLs and files:'
       echo