Fix Code Climate test coverage reports on wrong branch

GitHub Actions actions/checkout@v1 checked out code as a detached HEAD,
which caused Code Climate test coverage reports to appear as on the HEAD branch.

Coverage reports should now appear on an upstream branch.

From the actions/checkout@v2 release notes: https://github.com/actions/checkout/blob/v2/README.md

> - Creates a local branch
>   - No longer detached HEAD when checking out a branch
>   - A local branch is created with the corresponding upstream branch set

.github/workflows/test-unit.yml

@@ -41,8 +41,6 @@ jobs:
           --health-retries 5
 
     steps:
-    - uses: actions/checkout@v1
-
     - name: Update operating system software repository
       run: apt-get update
 
@@ -65,6 +63,8 @@ jobs:
     - name: Install PHP pdo_mysql extension
       run: docker-php-ext-install pdo_mysql
 
+    - uses: actions/checkout@v2
+
     - name: Install Composer
       run: curl -sSL https://getcomposer.org/installer | php -- --install-dir=/usr/local/bin/ --filename=composer