Update nix-installer-action version to v21

Author: josh

.github/workflows/nix.yml

@@ -20,7 +20,7 @@ jobs:
     runs-on: ${{ matrix.os }}
 
     steps:
-      - uses: DeterminateSystems/nix-installer-action@v3.13.2
+      - uses: DeterminateSystems/nix-installer-action@v21
         with:
           extra-conf: |
             accept-flake-config = true