Update cachix/cachix-action action to v15

Author: renovate[bot]

.github/workflows/deploy.yaml

@@ -17,10 +17,10 @@ jobs:
         install_options: '--tarball-url-prefix https://nixos-nix-install-tests.cachix.org/serve'
         extra_nix_config: |
           experimental-features = nix-command flakes
-    - uses: cachix/cachix-action@v10
+    - uses: cachix/cachix-action@v15
       with:
         name: iohk
-    - uses: cachix/cachix-action@v10
+    - uses: cachix/cachix-action@v15
       with:
         name: tinybeachthor
         signingKey: ${{ secrets.CACHIX_SIGNING_KEY }}