Snapcraft can't re-use build VM if anything went wrong with the initial setup

[Kyle Fazzari]

I made the mistake of trying build VMs from source, which installs snapcraft from the stable channel in the VM, which doesn't support managed-host as a build environment. This error looks like this:

$ SNAPCRAFT_BUILD_ENVIRONMENT=multipass snapcraft
Launching a VM.
Launched: snapcraft-my-snap-name
2018-09-26T09:54:29Z INFO Waiting for restart...
core 16-2.35 from 'canonical' installed
snapcraft 2.43.1 from Canonical✓ installed
SNAPCRAFT_BUILD_ENVIRONMENT must be one of: 'host' or 'lxd'.
An error occurred when trying to execute 'snapcraft refresh' with 'multipass': returned exit code 2.

Okay, so I installed a snap on my host that I knew would work, and ran again:

$ SNAPCRAFT_BUILD_ENVIRONMENT=multipass snapcraft
Launching a VM.
failed to launch: instance "snapcraft-my-snap-name" already exists
An error occurred when trying to launch the instance with 'multipass': returned exit code 2.
Ensure that 'multipass' is setup correctly and try again.

I need to either run `SNAPCRAFT_BUILD_ENVIRONMENT=multipass snapcraft clean` or blow away the instance manually using multipass in order to continue. Snapcraft should handle this situation better.