extensions-download-dir doesn't seem to respect 20 cache limit until restarting VS Code

[mjbvz]

Refs #2842

Repro

1. On MacOS using --extensions-download-dir
2. Download a bunch of extension packs

Bug
It looks like we just keep accumulating cached extensions until VS Code is restarted:

My expectation was that the 20 extension limit would also be enforced while VS Code is still running

[mjbvz]

Reading through #2842 again, this does seem by-design but it means that once you hit the 20 download limit, you will always have more than 20 extensions in that cache

[sandy081]

Yeah clean up is always done on start up.