[services][electron update server] Add patch-package to root postinstall

Summary: Currently patch-package is only run from the native workspace but it would make sense to run it at the end after all workspace members are correctly installed (in particular the new electron-update-server requires patches to the hazel package)

Test Plan: Run yarn cleaninstall and see that patch-package is run at the end

Reviewers: atul, tomek, ashoat

Reviewed By: tomek, ashoat

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D6391