fix: resolve --system flag issue

Author: Ritiz Tambi

src/art/skypilot/backend.py

@@ -197,7 +197,8 @@ async def _launch_cluster(
         elif os.path.exists(art_version):
             # copy the contents of the art_path onto the new machine
             task.workdir = art_version
-            art_installation_command = "uv sync --extra backend"
+            # uv sync doesn't support --system, so we use pip install in editable mode
+            art_installation_command = "uv pip install --system -e '.[backend]'"
         else:
             raise ValueError(
                 f"Invalid art_version: {art_version}. Must be a semver or a path to a local directory."