plume: update GCS URL to bincache URL

[tormath1]

• plume: remove BasePrivateURL
  it's no more used.

• plume/release: update GCS URL to bincache URL

---

I was expecting more complexity to update this URL but it seems that storage is able to understand both type of URL, it converts GCS URL to plain HTTP url:
https://github.com/flatcar-linux/mantle/blob/fb7bbf6e2c863c0bcfb9dcba4c090e45eb8c4742/cmd/plume/containerlinux.go#L326-L328

Related issue: flatcar/Flatcar#838

[pothos]

Thanks, one last missing piece is to split the ami txt/json file creation out of awsUploadAmiLists in a separate function which we then always call. There we could store them as local files. (awsUploadAmiLists would still be skipped when no GCS creds are given; the signing and copy to bincache is better done by the ci-automation bash code).

[pothos]

Thanks, one last missing piece is to split the ami txt/json file creation out of awsUploadAmiLists in a separate function which we then always call. There we could store them as local files. (awsUploadAmiLists would still be skipped when no GCS creds are given; the signing and copy to bincache is better done by the ci-automation bash code).

Done in #373