Add missing checkout step to github-release job (#146)

gh release create --verify-tag needs a local git checkout to verify
the tag against; the job never had one, so it would fail on any run,
not just the reuse_run_id retry path.

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

Author: philippjfr

.github/workflows/release.yml

@@ -249,6 +249,9 @@ jobs:
       actions: read
 
     steps:
+      - name: Checkout the tagged source
+        uses: actions/checkout@v6
+
       - name: Download Python distributions
         uses: actions/download-artifact@v4
         with: