netpbm: Update to 10.66.00

Also, borrow a tarball from Gentoo so we don't have to use
fetchsvn. And don't depend on libX11 by default.

(cherry picked from commit 08732891e1f4de4d03e103ec588da4c010f07370)
test-driver: Use netpbm instead of imagemagick

Imagemagick pulls in 100s of megabytes of dependencies.

(cherry picked from commit 51c349d0cc39fa8f692386e3f6d25653a91e17db)
Installer test: Use the minimal ISO

So we don't depend on KDE etc. (which we don't even test here).

(cherry picked from commit 0a967b926871af2eb8a5098d85a519a4922c56e7)
Add all default NixOS packages to the channel

The job ‘dummy’ depends on the default contents of
‘environment.systemPackages’, thus ensuring that those packages all
end up in the channel.

(cherry picked from commit 1cd727180eeb60e1c4f6ead96be614431624830f)
Add a jobset for a "nixos-small" channel

This channel only builds a small subset of Nixpkgs, mostly suitable
for servers. Since the channel update doesn't require thousands of
packages to be built first, it should provide much faster turnaround
in case of security updates.