missing lxd instructions should suggest snap

[Evan]

What happens:

$ snapcraft cleanbuild --debug
Starting snapcraft 2.27.1 from /snap/snapcraft/3/lib/python3.6/site-packages/snapcraft.
Traceback (most recent call last):
  File "/snap/snapcraft/3/bin/snapcraft", line 31, in <module>
    snapcraft.main.main()
  File "/snap/snapcraft/3/lib/python3.6/site-packages/snapcraft/main.py", line 244, in main
    return run(args, project_options)
  File "/snap/snapcraft/3/lib/python3.6/site-packages/snapcraft/main.py", line 286, in run
    lifecycle.cleanbuild(project_options, remote=args['--remote']),
  File "/snap/snapcraft/3/lib/python3.6/site-packages/snapcraft/internal/lifecycle.py", line 329, in cleanbuild
    'The lxd package is not installed, in order to use `cleanbuild` '
OSError: The lxd package is not installed, in order to use `cleanbuild` you must install lxd onto your system. Refer to the "Ubuntu Desktop and Ubuntu Server" section on https://linuxcontainers.org/lxd/getting-started-cli/#ubuntu-desktop-and-ubuntu-server to enable a proper setup.

What should happen:

$ snapcraft cleanbuild --debug
You need lxd installed to use cleanbuild. Please run `snap install lxd` and try again.

[Evan (ev)]

Duplicate of 1666735