Snap packages with classic confinement should recommend using `--classic`

[Alex Lowe]

Currently, attempting to run a command from a classic-confined snap goes as follows:

lengau@hyperion:~/Projects/cli$ gitkraken
Command 'gitkraken' not found, but can be installed with:
sudo snap install gitkraken
lengau@hyperion:~/Projects/cli$ sudo snap install gitkraken
[sudo] password for lengau:
error: This revision of snap "gitkraken" was published using classic confinement and thus may
       perform arbitrary system changes outside of the security sandbox that snaps are usually
       confined to, which may put your system at risk.

       If you understand and want to proceed repeat the command including --classic.
lengau@hyperion:~/Projects/cli$ sudo snap install gitkraken --classic
gitkraken 9.0.0 from gitkraken✓ installed
lengau@hyperion:~/Projects/cli$

Essentially, `command-not-found` is providing a command that requires a second attempt before it works. While this does make the user aware of the risks of classic-confined snaps, this can be done within the output of `command-not-found` instead if needed, still offering a better user experience.