[ci] Bump github actions runners

Signed-off-by: Douglas Reis <doreis@lowrisc.org>

Author: engdoreis

.github/workflows/ci.yml

@@ -22,12 +22,12 @@ jobs:
     runs-on: nixos
     steps:
       - name: checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           submodules: 'recursive'
 
       - name: Install Nix
-        uses: cachix/install-nix-action@v30
+        uses: cachix/install-nix-action@v31
 
       - name: Nix Format Check
         run: nix fmt -- . --check --exclude ./third_party --exclude ./cheriot-rtos