This adds the command `mach repackage snap`, which packages the current
build as a Snap (with the app name firefox_devel, to not interfere with
installs of the normal firefox package). It can also accept a tar file
as input, for example to repack a build from Taskcluster.
It also adds `mach repackage snap-install`, which installs the package
and connects its plugs with `snap connect` (normally done by default,
but not for an unsigned package like this).
Building a Snap package requires the `snapcraft` command, which is
available as a Snap (and maybe from other package managers).
Depends on D182615
Differential Revision: https://phabricator.services.mozilla.com/D182616
342132bb48