Get the correct icon name for package manager

[danirabbit]

sideload/src/Views/MainView.vala

Line 127 in 05789ba

var repo_icon = new Gtk.Image.from_icon_name ("system-software-install-symbolic", Gtk.IconSize.BUTTON);

We already get the correct app name (and thus app info), so we should make sure to pull the right icon name here as well