ci: Add action-editorconfig-checker to the GitHub Action for linting

The editorconfig-checker program is not installed by default in the OS
used by GitHub Actions. Fortunately, the editorconfig-checker authors
have kindly provided the action-editorconfig-checker repository, which
we are now using.

.github/workflows/lint.yml

@@ -23,6 +23,8 @@ jobs:
           restore-keys: ${{ runner.os }}-pip-
       - name: Install yamllint
         run: pip install yamllint
+      - name: Install editorconfig-checker
+        uses: editorconfig-checker/action-editorconfig-checker@main
       - name: Check out the code
         uses: actions/checkout@v4
       - name: Run the linting script