Merge pull request 'Update DeterminateSystems/nix-installer-action action to v21' (#217) from renovate/determinatesystems-nix-installer-action-21.x into main

.gitea/workflows/update.yml

@@ -15,7 +15,7 @@ jobs:
       - name: Checkout repository
         uses: actions/checkout@v5
       - name: Install nix
-        uses: DeterminateSystems/nix-installer-action@v20
+        uses: DeterminateSystems/nix-installer-action@v21
         with:
           github-token: ${{ secrets.GH_TOKEN_FOR_UPDATES }}
           extra_nix_config: |