fix: use Nix official installer with --no-daemon

The Determinate Systems installer doesn't support --no-daemon.
Switch to the official Nix installer which has a well-documented
--no-daemon flag suitable for container/CI use.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.gitea/workflows/deploy.yml

@@ -15,8 +15,7 @@ jobs:
 
       - name: Install Nix
         run: |
-          curl --proto '=https' --tlsv1.2 -sSf -L https://install.determinate.systems/nix \
+          sh <(curl -L https://nixos.org/nix/install) --no-daemon 2>&1
-            | sh -s -- install --no-confirm --no-daemon
           echo "$HOME/.nix-profile/bin" >> "$GITHUB_PATH"
 
       - name: Build with Nix