Refine checking for trailing whitespace [skip travis][skip appveyor]

* Include tabs in checking.
* Use `git grep` to avoid including `.git` directory.
* Avoid running `grep` multiple times.

.github/workflows/lint.yml

@@ -32,9 +32,7 @@ jobs:
     steps:
       - uses: actions/checkout@v2
       - name: 🧹 Check for trailing whitespace
-        run: |
-          cd test || exit
-          sh ./check-for-trailing-whitespace.sh || exit 1
+        run: "! git grep -EIn $'[ \t]+$'"
   yamllint:
     name: 🍶 YAML
     runs-on: ubuntu-latest