chore(deps): update determinatesystems/nix-installer-action action to v21

Author: renovate[bot]

.github/workflows/update-nix-flakes.yml

@@ -15,7 +15,7 @@ jobs:
       - uses: actions/checkout@v5
 
       - name: install nix
-        uses: DeterminateSystems/nix-installer-action@v20
+        uses: DeterminateSystems/nix-installer-action@v21
 
       - uses: actions/[email protected]
         id: generate-token