ci: avoid duplicating release notes on existing releases

[locker95]

Problem

Several cAdvisor GitHub releases contain duplicated release notes (e.g. What's Changed, PR list, Full Changelog appear twice):

• https://github.com/google/cadvisor/releases/tag/v0.60.2
• https://github.com/google/cadvisor/releases/tag/v0.60.1
• (and many other recent tags)

Root cause

The Release Binaries workflow uses softprops/action-gh-release with generate_release_notes: true.

That action appends freshly generated notes to any existing release body. Maintainers typically create the GitHub Release (with auto-generated notes, author = maintainer) before this workflow finishes building and uploads binaries. The create-release job then finds the existing release and regenerates notes, producing a duplicate body.

Timeline for v0.60.5 (same pattern on other tags):

1. Release created/published by maintainer with notes (~05:07–05:08)
2. Tag push starts the workflow (~05:08)
3. create-release job runs later (~05:18) and softprops appends notes again
4. Assets are uploaded by github-actions[bot]

Fix

Before calling softprops, check whether the release already has a body:

• Body present → generate_release_notes: false (only attach artifacts)
• No release / empty body → generate_release_notes: true (generate notes once)

Test

Workflow YAML only; verified against release timeline and softprops update path (prepareReleaseMutation concatenates generated notes onto an existing body).

Fixes #3901