fix task leak (microsoft#262732)

fix microsoft#252515

Author: meganrogge

src/vs/workbench/contrib/tasks/browser/terminalTaskSystem.ts

@@ -1538,7 +1538,10 @@ export class TerminalTaskSystem extends Disposable implements ITaskSystem {
 		}
 		const terminalKey = terminal.instanceId.toString();
 		const terminalData = { terminal: terminal, lastTask: taskKey, group };
-		terminal.onDisposed(() => this._deleteTaskAndTerminal(terminal, terminalData));
+		const onDisposedListener = this._register(terminal.onDisposed(() => {
+			this._deleteTaskAndTerminal(terminal, terminalData);
+			onDisposedListener.dispose();
+		}));
 		this._terminals[terminalKey] = terminalData;
 		terminal.shellLaunchConfig.tabActions = this._terminalTabActions;
 		return [terminal, undefined];