fix: forward NOMAD_TOKEN to $GITHUB_ENV for shell access

Job-level env vars from secrets aren't injected as real shell env
vars in Docker-based composite actions. Bridge via $GITHUB_ENV,
same pattern as NOMAD_ADDR.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

deploy-nix-site/action.yaml

@@ -29,8 +29,9 @@ runs:
         # Install AWS CLI
         nix profile install nixpkgs#awscli2
 
-        # Set Nomad address
+        # Set Nomad connection
         echo "NOMAD_ADDR=http://alvin:4646" >> $GITHUB_ENV
+        echo "NOMAD_TOKEN=${{ env.NOMAD_TOKEN }}" >> $GITHUB_ENV
 
     - name: Build site with Nix
       shell: bash