Update cachix/install-nix-action to version 31

Author: hugtalbot

.github/workflows/nix.yml

@@ -11,7 +11,7 @@ jobs:
         os: [ubuntu, macos]
     steps:
       - uses: actions/checkout@v4
-      - uses: cachix/install-nix-action@v27
+      - uses: cachix/install-nix-action@v31
       #- uses: cachix/cachix-action@v15
         #with:
           #name: sofa