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

Author: renovate[bot]

.github/workflows/ci.yml

@@ -25,7 +25,7 @@ jobs:
 
     steps:
       - uses: actions/checkout@v6
-      - uses: DeterminateSystems/nix-installer-action@v20
+      - uses: DeterminateSystems/nix-installer-action@v21
         with:
           diagnostic-endpoint: ''
 
@@ -49,7 +49,7 @@ jobs:
 
     steps:
       - uses: actions/checkout@v6
-      - uses: DeterminateSystems/nix-installer-action@v20
+      - uses: DeterminateSystems/nix-installer-action@v21
         with:
           diagnostic-endpoint: ''