ci: install nix manually instead of using action

The DeterminateSystems/nix-installer-action requires Node 18+ but the
Gitea act runner uses Node 16. Use curl installer directly instead.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.gitea/workflows/flake-update.yaml

@@ -11,7 +11,10 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - uses: DeterminateSystems/nix-installer-action@main
+      - name: Install Nix
+        run: |
+          curl --proto '=https' --tlsv1.2 -sSf -L https://install.determinate.systems/nix | sh -s -- install linux --no-confirm
+          echo "/nix/var/nix/profiles/default/bin" >> "$GITHUB_PATH"
 
       - name: Update flake inputs
         run: nix flake update