ci/*: bump nix

Author: PedroHLC

.github/workflows/document.yml

@@ -25,6 +25,7 @@ jobs:
         uses: cachix/install-nix-action@v23
         with:
           extra_nix_config: "accept-flake-config = true"
+          install_url: https://releases.nixos.org/nix/nix-2.18.1/install
       - name: Build
         id: build
         run: |

.github/workflows/flakehub-publish-rolling.yml

@@ -20,6 +20,7 @@ jobs:
             accept-flake-config = true
             sandbox-fallback = false
           install_options: --daemon
+          install_url: https://releases.nixos.org/nix/nix-2.18.1/install
       - uses: "actions/checkout@v4"
       - name: Update Nix
         run : nix profile install .#nix-flake-schemas_git

.github/workflows/needs-cache.yml

@@ -22,6 +22,7 @@ jobs:
             accept-flake-config = true
             sandbox-fallback = false
           install_options: --daemon
+          install_url: https://releases.nixos.org/nix/nix-2.18.1/install
       - name: Checkout code
         uses: actions/checkout@v4
         with:

.github/workflows/nixpkgs-bump.yml

@@ -24,6 +24,7 @@ jobs:
         uses: cachix/install-nix-action@v23
         with:
           extra_nix_config: "accept-flake-config = true"
+          install_url: https://releases.nixos.org/nix/nix-2.18.1/install
       - uses: actions/checkout@v4
       - name: Update nixpkgs flake
         run: nix flake lock --update-input nixpkgs || exit 1; git diff --exit-code flake.lock && exit 1 || exit 0