ci: platform binary artifacts in build job missing retention-days

[Ankitsinghsisodya]

The build job uploads 7 cross-platform binaries (darwin/amd64, darwin/arm64, linux/amd64, linux/arm64, linux/ppc64le, linux/s390x, windows/amd64) on every push to main with no retention-days setting, so they use GitHub's default 90-day retention.

Cluster-log artifacts in the same file already use retention-days: 7. Adding retention-days: 30 to binary artifacts aligns retention policy and avoids unbounded storage accumulation.

[Ankitsinghsisodya]

/assign