CI haskell.yml: bump actions/checkout to v5

Author: andreasabel

.github/workflows/haskell.yml

@@ -18,7 +18,7 @@ jobs:
         ghc: ['9.12', '9.10', '9.8', '9.6']
         os: [ubuntu-latest, macOS-latest, windows-latest]
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v5
 
     - uses: haskell-actions/setup@v2
       id:   setup