Cleanup apt cache after grub install

Grub installation happens in finalize.d so cleanup should be done after this.

This reapplies I6322b8b529e31fd1b7251dd9e07cc85f442f7e81 after the revert,
but working.

Change-Id: Ie1873e64d407552da37a8dbedc13c3adbf79c085
Merge "Cleanup apt cache after grub install"