tee output to console

This should hopefully help in debugging. It's a bit gnarly to do this
so it relies on some redirection magic. It also needed to be on a new
line to pass the linter (which otherwise doesn't think it's a string).

https://serverfault.com/a/989827

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

action.yml

@@ -20,5 +20,6 @@ runs:
       env:
         GITHUB_ACTION_PATH: ${{ github.action_path }}
 
-    - run: bin/diff-check >> $GITHUB_STEP_SUMMARY
+    - run: >
+        { GITHUB_STEP_SUMMARY=$(bin/diff-check | tee /dev/fd/3); } 3>&1
       shell: bash