fix: add trailing slash to tag ref prefix in Git._clone (#10799)

The tag import prefix was b"refs/tags" (no trailing slash), so
stripping it from e.g. b"refs/tags/v1.0" produced b"/v1.0" instead
of b"v1.0", creating malformed local refs.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: dimbleby

src/poetry/vcs/git/backend.py

@@ -373,7 +373,7 @@ def _clone(cls, url: str, refspec: GitRefSpec, target: Path) -> Repo:
 
         for base, prefix in {
             (Ref(b"refs/remotes/origin"), b"refs/heads/"),
-            (Ref(b"refs/tags"), b"refs/tags"),
+            (Ref(b"refs/tags"), b"refs/tags/"),
         }:
             try:
                 local.refs.import_refs(