fix: install Nix in CI before building

The Gitea Actions runner image (docker.gitea.com/runner-images:ubuntu-latest)
does not have Nix pre-installed, causing 'nix: command not found' on the
build step.

Add an 'Install Nix' step using the Determinate Systems installer
(--no-daemon mode since the runner runs in a container).

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

.gitea/workflows/deploy.yml

@@ -13,8 +13,15 @@ jobs:
       - name: Check out repository
         uses: actions/checkout@v4
 
+      - name: Install Nix
+        run: |
+          curl --proto '=https' --tlsv1.2 -sSf -L https://install.determinate.systems/nix \
+            | sh -s -- install --no-confirm --no-daemon
+          echo "$HOME/.nix-profile/bin" >> "$GITHUB_PATH"
+
       - name: Build with Nix
         run: |
+          . "$HOME/.nix-profile/etc/profile.d/nix.sh"
           nix build .#teleuploader --impure --option sandbox false 2>&1
 
       - name: Deploy (local — runner is on same VPS)