[snapcraft 3.x] Cross-compilation amd64 → armhf in Multipass VM

[tobias]

@sergiusens cleared that up over in @eberkund’s thread:

Snapcraft 3.x target-arch not working

We decided not to move with --target-arch yet when using bases as we want to have an improved experience. Sorry for the disruption, but it should be possible to use this with --destructive-mode still. The reason for moving out of this is that there are many corner cases presented with cross compilation and the experience is not that great for general use.