build: Generate SBOM during image release

[joestringer]

This reverts commit b7cd73e.

SBOM was a focus of some of the problems around the upgrade to docker
buildx v0.10 which introduces various build failures across the Cilium
CI. It was reverted to try to get CI back in a stable state. This was
unsuccessful because the root cause was not related to these workflow
changes, rather these steps add a downstream consumer of the docker
images and the docker image format changed. While reverting these steps
did prevent them from breaking CI, there were other parts of CI that
also still broke due to the image format change. Given that these CI
steps were running successfully for the better part of the last week
already and they weren't the root cause of the recent CI instability, I
think it makes sense to restore this feature into CI.

Reverts: #23204

[joestringer]

This failed with the image sha not being passed to the signing step:

https://github.com/cilium/cilium/actions/runs/3971027337/jobs/6807456125#step:16:19

Run cosign sign quay.io/cilium/docker-plugin-ci@
  cosign sign quay.io/cilium/docker-plugin-ci@
  cosign sign quay.io/cilium/docker-plugin-ci@
  cosign sign quay.io/cilium/docker-plugin-ci@
  shell: /usr/bin/bash -e {0}
  env:
    QUAY_ORGANIZATION: cilium
    QUAY_ORGANIZATION_DEV: cilium
    MSYS: winsymlinks:nativestrict
    COSIGN_EXPERIMENTAL: true
Generating ephemeral keys...
Retrieving signed certificate...

        Note that there may be personally identifiable information associated with this signed artifact.
        This may include the email address associated with the account with which you authenticate.
        This information will be used for signing this artifact and will be stored in public transparency logs and cannot be removed later.
Successfully verified SCT...
Error: signing [quay.io/cilium/docker-plugin-ci@]: parsing reference: could not parse reference: quay.io/cilium/docker-plugin-ci@
main.go:6[2](https://github.com/cilium/cilium/actions/runs/3971027337/jobs/6807456125#step:16:2): error during command execution: signing [quay.io/cilium/docker-plugin-ci@]: parsing reference: could not parse reference: quay.io/cilium/docker-plugin-ci@
Error: Process completed with exit code 1.

[aanm]

Merging since this PR was verified with the test commit 1dd46665933175fb4d4a0719e3b4f5e6476cb8d0

COSIGN_EXPERIMENTAL=1 cosign verify --certificate-github-workflow-repository cilium/cilium --certificate-oidc-issuer https://token.actions.githubusercontent.com --attachment sbom  quay.io/cilium/docker-plugin-ci:1dd46665933175fb4d4a0719e3b4f5e6476cb8d0 | jq