CI: update Cachix action

Author: aspiwack

.github/workflows/ci.yaml

@@ -15,7 +15,7 @@ jobs:
     - uses: cachix/install-nix-action@v15
       with:
         nix_path: "${{ env.nixpkgs-url }}"
-    - uses: cachix/cachix-action@v12
+    - uses: cachix/cachix-action@v15
       with:
         name: aspiwack
         # If you chose API tokens for write access OR if you have a private cache
@@ -33,7 +33,7 @@ jobs:
     - uses: cachix/install-nix-action@v15
       with:
         nix_path: "${{ env.nixpkgs-url }}"
-    - uses: cachix/cachix-action@v12
+    - uses: cachix/cachix-action@v15
       with:
         name: aspiwack
         # If you chose API tokens for write access OR if you have a private cache