[wip] release: sync alpha bincache assets to s3 bucket

[tormath1]

In this PR, we add a new step to the 'release' CI job. The goal is to publish release artifacts for alpha channel on Cloudflare Bucket to experiment with this kind of storage for release artifacts.

TODO:

• Add r2 config as secret
• Setup r2.dev URL for public development URL

Locally tested:

$ aws s3 ls s3://flatcar/ --recursive
2025-08-12 10:26:44       1237 alpha/amd64/4372.0.1/flatcar-container.tar.gz.DIGESTS.asc
2025-08-12 10:26:45        594 alpha/amd64/4372.0.1/flatcar-container.tar.gz.DIGESTS.sig
2025-08-12 10:26:45        318 alpha/amd64/4372.0.1/flatcar-container.tar.gz.DIGESTS
2025-08-12 10:26:44        594 alpha/amd64/4372.0.1/flatcar-container.tar.gz.sig
2025-08-12 10:34:12  409256107 alpha/amd64/4372.0.1/flatcar-container.tar.gz
2025-08-12 10:26:46       1216 alpha/amd64/4372.0.1/flatcar-incus.raw.DIGESTS.asc
2025-08-12 10:26:46        594 alpha/amd64/4372.0.1/flatcar-incus.raw.DIGESTS.sig
2025-08-12 10:26:46        297 alpha/amd64/4372.0.1/flatcar-incus.raw.DIGESTS
2025-08-12 10:26:47        594 alpha/amd64/4372.0.1/flatcar-incus.raw.sig
2025-08-12 10:38:02  122224640 alpha/amd64/4372.0.1/flatcar-incus.raw
...

Needs:

• docker: add rclone mantle#649