chore(deps): update cachix/cachix-action action to v16

Author: renovate[bot]

.github/workflows/nix.yml

@@ -16,7 +16,7 @@ jobs:
     steps:
       - uses: actions/checkout@v5
       - uses: cachix/install-nix-action@v31
-      - uses: cachix/cachix-action@v15
+      - uses: cachix/cachix-action@v16
         with:
           name: numtide
           authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
@@ -36,7 +36,7 @@ jobs:
           # Nix Flakes doesn't work on shallow clones
           fetch-depth: 0
       - uses: cachix/install-nix-action@v31
-      - uses: cachix/cachix-action@v15
+      - uses: cachix/cachix-action@v16
         with:
           name: numtide
           authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
@@ -60,7 +60,7 @@ jobs:
     steps:
       - uses: actions/checkout@v5
       - uses: cachix/install-nix-action@v31
-      - uses: cachix/cachix-action@v15
+      - uses: cachix/cachix-action@v16
         with:
           name: numtide
           authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'