ci: pass --init none to nix installer for Docker container

The act runner uses Docker containers without systemd, so the Nix
installer needs --init none for a root-only install.

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

.gitea/workflows/flake-update.yaml

@@ -13,7 +13,7 @@ jobs:
 
       - name: Install Nix
         run: |
-          curl --proto '=https' --tlsv1.2 -sSf -L https://install.determinate.systems/nix | sh -s -- install linux --no-confirm
+          curl --proto '=https' --tlsv1.2 -sSf -L https://install.determinate.systems/nix | sh -s -- install linux --no-confirm --init none
           echo "/nix/var/nix/profiles/default/bin" >> "$GITHUB_PATH"
 
       - name: Update flake inputs