Isolate PartyKit deploy auth env

[agustin-lowpoly]

Summary

• unset Cloudflare R2 env vars for the PartyKit deploy step
• keep R2 upload env scoped away from PartyKit auth selection

Validation

• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/deploy.yml"); puts "deploy-yaml-ok"'\n- git diff --check -- .github/workflows/deploy.yml\n\n## Root cause\nThe Deploy job sets Cloudflare R2 credentials globally. PartyKit CLI sees CLOUDFLARE_ACCOUNT_ID / CLOUDFLARE_API_TOKEN and takes the Cloudflare Workers auth route instead of the PartyKit token route. That is why local deploy with PartyKit token worked while GitHub Actions failed with Authentication error.