ci: Update cachix action to fix deprecation warning (#43)

Author: utdemir

.github/workflows/build.yaml

@@ -14,7 +14,7 @@ jobs:
       with:
         extra_nix_config: |
           experimental-features = nix-command flakes   
-    - uses: cachix/cachix-action@v11
+    - uses: cachix/cachix-action@v12
       with:
         name: utdemir
         signingKey: '${{ secrets.CACHIX_SIGNING_KEY }}'