Remove Feedz publishing from CI

[meziantou]

What changed

Removes the Feedz feed from the deploy job of .github/workflows/ci.yml:

• The Publish NuGet packages step no longer branches on GITHUB_REF. It always pushes to nuget.org, and the whole step is gated on github.ref == 'refs/heads/main' — previously the else branch (non-main) is what pushed to https://f.feedz.io/meziantou/meziantou-analyzer/nuget/index.json.
• Dropped the FeedzApiKey env var and its secrets.FEEDZ_APIKEY usage.
• Gated the NuGet login (OIDC → temp API key) step on main as well, since its only consumer is the push step. It was previously minting a temporary nuget.org API key on every non-fork PR run with nothing left to use it.

Why

Feedz publishing of CI builds is no longer wanted.

Notes for reviewers

• The deploy job itself still runs on PRs (it checks out, downloads the nuget artifact and sets up .NET), so any required status check on deploy keeps reporting instead of being skipped. Both remaining publish-related steps simply no-op off main.
• No source or test code changed, so there was nothing to build or run; the workflow file parses as valid YAML and no feedz reference remains in the repo.
• The FEEDZ_APIKEY repository secret is now unused and can be deleted in the GitHub settings.