ci: name the tag in GitHub's tag status, and make digests re-runnable

Commit statuses belong to the commit, not the tag. Upstream v* tags and
inbuxa-v* tags can sit on the same commit, so Gitea's github job, waiting
for "github/ci (tag)", could read the other tag's older result and move on
before this tag's build finished. A tag's status is now "github/ci (tag
<name>)" on both sides.

The per-architecture digest artifacts are also overwritable and kept for a
week, so re-running a build, or publish on its own, still works.

.gitea/workflows/ci.yml

@@ -228,7 +228,7 @@ jobs:
     env:
       TOKEN: ${{ secrets.GITHUB_TOKEN }}
       SHA: ${{ github.event.pull_request.head.sha || github.sha }}
-      CONTEXT: github/ci (${{ github.ref_type == 'tag' && 'tag' || 'branch' }})
+      CONTEXT: github/ci (${{ github.ref_type == 'tag' && format('tag {0}', github.ref_name) || 'branch' }})
     steps:
       - run: apt-get update -qq && apt-get install -y -qq --no-install-recommends ca-certificates curl jq >/dev/null
       - shell: bash