tee output to console (#6)

This should hopefully help in debugging. `$GITHUB_STEP_SUMMARY` is an
"environment file" provided by GitHub Actions, so we can treat it just
like a file and use `tee` to append data to it.

Based on an idea by @oscargus raised in #4.

https://docs.github.com/en/actions/writing-workflows/choosing-what-your-workflow-does/workflow-commands-for-github-actions#adding-a-job-summary
https://docs.github.com/en/actions/writing-workflows/choosing-what-your-workflow-does/workflow-commands-for-github-actions#environment-files

action.yml

@@ -14,7 +14,7 @@ runs:
     - run: ${{ inputs.command }}
       shell: bash
 
-    - run: $GITHUB_ACTION_PATH/bin/diff-check >> $GITHUB_STEP_SUMMARY
+    - run: $GITHUB_ACTION_PATH/bin/diff-check | tee -a $GITHUB_STEP_SUMMARY
       shell: bash
       env:
         GITHUB_ACTION_PATH: ${{ github.action_path }}