zoom-us: Missing desktop icon

[tadfisher]

Describe the bug

zoom-us is missing its icon, because it does not appear in the source tarball.

I see a couple of options to fix:

1. Fetch the icon separately, perhaps from https://github.com/flathub/us.zoom.Zoom
2. Change the source to one of the packaged releases for another distro that includes the icon

cc @danbst

Maintainer information:

# a list of nixpkgs attributes affected by the problem
attribute: zoom-us
# a list of nixos modules affected by the problem
module:

[worldofpeace]

I would install the desktop file and appdata from there as well 👍