Switch to using a bash script. (#39)

.github/workflows/test.yml

@@ -17,6 +17,8 @@ jobs:
         uses: actions/checkout@v2
         with:
           fetch-depth: 0
+      - name: shellcheck
+        uses: reviewdog/action-shellcheck@v1
       - name: Run changed-files with defaults
         id: changed-files
         uses: ./