stylish-haskell: check Haskell source files only

The style check was failing due to a changed stack.yaml.lock file.

(That file still should be updated, but this is not where it should
cause a failure.)

.circleci/config.yml

@@ -112,7 +112,7 @@ jobs:
           command: git ls-files | grep '\.l\?hs$' | xargs stack exec -- hlint -X QuasiQuotes -X NoPatternSynonyms "$@"
       - run:
           name: run styler
-          command: git ls-files | grep '\.l\?hs$' | xargs stack exec -- stylish-haskell -i && git diff-index --exit-code HEAD --
+          command: git ls-files | grep '\.l\?hs$' | xargs stack exec -- stylish-haskell -i && git diff-index --exit-code HEAD -- '*.hs' '*.lhs'
 
   build-test-9.6:
     docker: