snap: add track 'latest' if no track is specified (#6835)
* snap: add track 'latest' if no track is specified
See https://snapcraft.io/docs/channels for more details.
* snap: assume track latest if channel does not specify it
(cherry picked from commit dc0d00452f)
Co-authored-by: Alexei Znamensky <103110+russoz@users.noreply.github.com>