Upgrade actions/checkout to v4

This will fix the following warning:

"The following actions uses node12 which is deprecated and will be
forced to run on node16: actions/checkout@v2. [...]"

.github/workflows/test.yml

@@ -26,7 +26,7 @@ jobs:
       run: curl -fsSL https://raw.github.com/doublep/eldev/master/webinstall/github-eldev | sh
 
     - name: Check out the source code
-      uses: actions/checkout@v2
+      uses: actions/checkout@v4
 
     - name: Test the project
       run: |