ci: try using iohk cachix

Author: thomasjm

.github/workflows/ci.yml

@@ -16,7 +16,10 @@ jobs:
 
     - uses: cachix/install-nix-action@v31
       with:
-        nix_path: nixpkgs=channel:nixos-unstable
+        nix_path: nixpkgs=channel:release-25.11
+        extra_nix_config: |
+          trusted-public-keys = iohk.cachix.org-1:DpRUyj7h7V830dp/i6Nti+NEO2/nhblbov/8MW7Rqoo= cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY=
+          substituters = https://iohk.cachix.org https://cache.nixos.org/
     - uses: DeterminateSystems/magic-nix-cache-action@main
 
     - uses: actions/cache@v5
@@ -55,7 +58,10 @@ jobs:
 
     - uses: cachix/install-nix-action@v31
       with:
-        nix_path: nixpkgs=channel:nixos-unstable
+        nix_path: nixpkgs=channel:release-25.11
+        extra_nix_config: |
+          trusted-public-keys = iohk.cachix.org-1:DpRUyj7h7V830dp/i6Nti+NEO2/nhblbov/8MW7Rqoo= cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY=
+          substituters = https://iohk.cachix.org https://cache.nixos.org/
     - uses: DeterminateSystems/magic-nix-cache-action@main
 
     - name: No GMP
@@ -73,7 +79,10 @@ jobs:
 
     - uses: cachix/install-nix-action@v31
       with:
-        nix_path: nixpkgs=channel:nixos-unstable
+        nix_path: nixpkgs=channel:release-25.11
+        extra_nix_config: |
+          trusted-public-keys = iohk.cachix.org-1:DpRUyj7h7V830dp/i6Nti+NEO2/nhblbov/8MW7Rqoo= cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY=
+          substituters = https://iohk.cachix.org https://cache.nixos.org/
     - uses: DeterminateSystems/magic-nix-cache-action@main
 
     - name: Build