snapcraft use cached packages across VMs

[keshavbhatt]

snapcraft not using a central store to cache packages when building inside VMs. Which requires downloading same package again and again to build snaps.
This particularly happen when this bug(https://bugs.launchpad.net/snapcraft/+bug/1869542) of snapcraft is triggered and there is no way to pass through it without using `snapcraft clean` command (which will delete the VM). This clears up the complete VM and forgets all downloaded packages by apt.