ci: update format check command to ignore .prettierignore files

This is to try and fix formatting issues on #773 on a file
that should be ignored.

.github/workflows/ci.yml

@@ -24,7 +24,7 @@ jobs:
       - name: Lint
         run: yarn lint
       - name: Formatting
-        run: yarn format:check
+        run: yarn format:check --ignore-path .prettierignore
       - name: Build
         run: yarn build