bug: expand {runtime} in plugin install paths sent to nodes

[lukepolo]

Plugins whose registry `install_path` uses `{runtime}` (Arenas) were extracted into a literal `addons/{runtime}/plugins` directory on the node.

• `desiredForNode` now substitutes the node's resolved runtime into `installPath`, matching how config paths are already handled
• Adds a regression test for the expanded path
• Nodes that already installed the broken path need the plugin removed and re-added, since the connector only reinstalls on a version change