Make release process compatible with immutable releases

[tonyandrewmeyer]

The current release process does not work when immutable release is set:

scm releases
      • token type                                   type=github
      • releasing                                    tag=v1.4.3 repo=canonical/concierge
      • refreshing checksums                         file=checksums.txt
      • checksumming                                 file=concierge_1.4.3_linux_s390x.tar.gz
      • calculating checksum for dist/concierge_1.4.3_linux_s390x.tar.gz
      • checksumming                                 file=concierge_1.4.3_linux_ppc64le.tar.gz
      • calculating checksum for dist/concierge_1.4.3_linux_ppc64le.tar.gz
      • checksumming                                 file=concierge_1.4.3_linux_arm64.tar.gz
      • calculating checksum for dist/concierge_1.4.3_linux_arm64.tar.gz
      • checksumming                                 file=concierge_1.4.3_linux_amd64.tar.gz
      • calculating checksum for dist/concierge_1.4.3_linux_amd64.tar.gz
      • release updated                              name=v1.4.3 release-id=302907151 request-id=BBC0:188F2:47C4F1:1257577:69C9903D
      • created                                      url=https://github.com/canonical/concierge/releases/tag/v1.4.3
      • uploading to release                         name=concierge_1.4.3_linux_s390x.tar.gz
      • uploading to release                         name=concierge_1.4.3_linux_ppc64le.tar.gz
      • uploading to release                         name=concierge_1.4.3_linux_arm64.tar.gz
      • uploading to release                         name=concierge_1.4.3_linux_amd64.tar.gz
      • upload failed                                request-id=BBC3:3D780F:19749:4E0717:69C9903E error=POST https://uploads.github.com/repos/canonical/concierge/releases/302907151/assets?name=concierge_1.4.3_linux_amd64.tar.gz: 422 Cannot upload assets to an immutable release. [] name=concierge_1.4.3_linux_amd64.tar.gz release-id=302907151
      • uploading to release                         name=checksums.txt
      • upload failed                                request-id=BBC3:3D780F:19749:4E0713:69C9903E error=POST https://uploads.github.com/repos/canonical/concierge/releases/302907151/assets?name=concierge_1.4.3_linux_ppc64le.tar.gz: 422 Cannot upload assets to an immutable release. [] name=concierge_1.4.3_linux_ppc64le.tar.gz release-id=302907151
      • upload failed                                request-id=BBC3:3D780F:19749:4E0718:69C9903E error=POST https://uploads.github.com/repos/canonical/concierge/releases/302907151/assets?name=concierge_1.4.3_linux_arm64.tar.gz: 422 Cannot upload assets to an immutable release. [] name=concierge_1.4.3_linux_arm64.tar.gz release-id=302907151
      • upload failed                                request-id=BBC3:3D780F:19749:4E0716:69C9903E error=POST https://uploads.github.com/repos/canonical/concierge/releases/302907151/assets?name=concierge_1.4.3_linux_s390x.tar.gz: 422 Cannot upload assets to an immutable release. [] name=concierge_1.4.3_linux_s390x.tar.gz release-id=302907151
      • upload failed                                request-id=BBC3:3D780F:1974C:4E0725:69C9903E error=POST https://uploads.github.com/repos/canonical/concierge/releases/302907151/assets?name=checksums.txt: 422 Cannot upload assets to an immutable release. [] name=checksums.txt release-id=302907151
    • took: 58s
  ⨯ release failed after 2m2s                        error=scm releases: failed to publish artifacts: failed to upload concierge_1.4.3_linux_amd64.tar.gz: POST https://uploads.github.com/repos/canonical/concierge/releases/302907151/assets?name=concierge_1.4.3_linux_amd64.tar.gz: 422 Cannot upload assets to an immutable release. []

This seems to be because the release adds in extra assets as part of the workflow, and as well as preventing changes to assets, GitHub also prevents adding new assets.

It would be nice if we could support this somehow -- maybe the assets can be uploaded first somehow?