Bump to actions/checkout@v4

because we get the warning:

The following actions uses node12 which is deprecated and will be
forced to run on node16: actions/checkout@v2. For more info:
https://github.blog/changelog/2023-06-13-github-actions-all-actions-will-run-on-node16-instead-of-node12-by-default/

.github/workflows/tests.yml

@@ -49,7 +49,7 @@ jobs:
 
     steps:
       - name: Clone project
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
 
       # This does not seem to cache on Windows, because ~/.stack does
       # not exist on Windows. I don't know how caching on Windows is