release.yml: set explicit artifact retention-days (linux)

[simonpainter]

In .github/workflows/release.yml we upload a build artifact:

• name: Upload artifact
  uses: actions/upload-artifact@v4
  with:
  name: uping-linux
  path: uping

Consider setting an explicit retention-days for these artifacts. Without it, artifacts are retained for the default duration, which can increase storage usage over time with frequent pushes to main.

Suggested fix

• Add e.g. retention-days: 1 (or another small, intentional value) to the upload-artifact step.

Acceptance

• Artifact retention is explicit and matches intended lifecycle/cost.