build(deps): bump DeterminateSystems/nix-installer-action from 20 to 21 (#394)

Author: dependabot[bot]

.github/workflows/update.yml

@@ -14,7 +14,7 @@ jobs:
     steps:
       - uses: actions/checkout@v5
       - name: Install Nix
-        uses: DeterminateSystems/nix-installer-action@v20
+        uses: DeterminateSystems/nix-installer-action@v21
         with:
           github-token: ${{ secrets.GITHUB_TOKEN }}
       - name: Update flake.lock