debug: activate auto launch onStartupFinished (#164481)

For https://github.com/microsoft/vscode-internalbacklog/issues/3209

We still need to activate it for every workspace, but since Daniel's
improvements to environment variable application a few months ago, using
onStartupFinished should be pretty safe and seamless.

extensions/debug-auto-launch/package.json

@@ -16,7 +16,7 @@
     }
   },
   "activationEvents": [
-    "*"
+    "onStartupFinished"
   ],
   "main": "./out/extension",
   "scripts": {