[unified_analytics] Fix getEnvironment() to write DashTool.label and use resolveRootTool #2702

Depends on #2700.

getEnvironment() writes DashTool.name (e.g. vscodePlugins), but every reader parses the value with DashTool.fromLabel (which expects vscode-plugins).

• Write .label instead.
• Use resolveRootTool(tool: currentTool), so a shared component that starts a child process passes on its root tool, not itself. Leave DASH__TOOL out when rootTool is null.

Done when: child processes receive a valid label that names a top-level tool.