Upload postcli release to GCP

Instead of appending the built `postcli` binary to the github release upload the binary to GCP and add links to the GH release, similar to how go-spacemesh handles it releases: https://github.com/spacemeshos/go-spacemesh/blob/a005d4c257a4e55b01ab71300acfd1e0f0c4774b/.github/workflows/release.yml#L99-L104