stage.py: improve path to url (spack#47898)

Author: haampie

lib/spack/spack/stage.py

@@ -488,7 +488,7 @@ def _generate_fetchers(self, mirror_only=False) -> Generator["fs.FetchStrategy",
             # Insert fetchers in the order that the URLs are provided.
             fetchers[:0] = (
                 fs.from_url_scheme(
-                    url_util.join(mirror.fetch_url, self.mirror_layout.path),
+                    url_util.join(mirror.fetch_url, *self.mirror_layout.path.split(os.sep)),
                     checksum=digest,
                     expand=expand,
                     extension=extension,