Don't re-draft an already-published release on every main push

stage_release runs on every push to main and unconditionally deletes
and recreates the draft release for the version in _version.py. Since
_version.py isn't bumped until sometime after a release is tagged and
published by release.yml, every intervening push to main was clobbering
the just-published release back to draft status. Confirmed this hit
v17.3.0 through v17.8.0 (and briefly v17.8.1, manually fixed).

Skip the delete/recreate if a non-draft release already exists for
this tag.

.github/workflows/build.yml

@@ -275,6 +275,14 @@ jobs:
         run: |
           TAG="v${{ steps.version.outputs.version }}"
 
+          # If release.yml already published this version, _version.py may
+          # still reflect it until the next version bump commit. Don't
+          # re-draft an already-published release on later pushes to main.
+          if [[ "$(gh release view "$TAG" --json isDraft --jq .isDraft 2>/dev/null)" == "false" ]]; then
+            echo "Release $TAG is already published; skipping."
+            exit 0
+          fi
+
           # Delete existing draft release if it exists (ignore errors)
           gh release delete "$TAG" --yes 2>/dev/null || true