feat: set `serverInstallPath` for the `<host>` automatically to the workspace dir

In some cases the $HOME dir is ephemeral, so when a workspace is restarted the `~/.vscode-server` folder is lost and all of the extensions have to be installed from scratch.

But adding an entry to the `settings.json` to persistence volume would cache the installled extensions between restarts.

```json5
"remote.SSH.serverInstallPath": {
"": "/workspace/"
}
```