Bump actions/checkout to v6

Author: stevepiercy

.github/workflows/build.yml

@@ -14,7 +14,7 @@ jobs:
           - python: python310
             sphinx: 8.2.3
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
     - uses: cachix/install-nix-action@v30
       with:
         nix_path: nixpkgs=channel:nixos-unstable