ci(release): ignore buildkitd.toml dropped by Blacksmith builder (#955)

## Summary

- `useblacksmith/setup-docker-builder` drops `buildkitd.toml` in the
repo
  root, which trips goreleaser's dirty-state check. v2.5.0 and v2.6.0
  shipped with no release artifacts for this reason.
- Adds a workflow step after the builder setup that writes the file's
  name to `.git/info/exclude`, so both `release:published` runs and
  `workflow_dispatch` backfills of older tags see a clean working tree.

Author: cpcloud

.github/workflows/release.yml

@@ -53,6 +53,13 @@ jobs:
       - name: Setup Blacksmith Builder
         uses: useblacksmith/setup-docker-builder@ac083cc84672d01c60d5e8561d0a939b697de542 # v1
 
+      # useblacksmith/setup-docker-builder drops buildkitd.toml in the repo
+      # root, which trips goreleaser's dirty-state check. Exclude it locally
+      # so both release:published runs and workflow_dispatch backfills of
+      # older tags see a clean working tree.
+      - name: Ignore buildkitd.toml locally
+        run: echo buildkitd.toml >> .git/info/exclude
+
       - uses: docker/login-action@4907a6ddec9925e35a0a9e82d7399ccc52663121 # v4.1.0
         with:
           registry: ${{ env.REGISTRY }}