Motivation for this change

Had someone on IRC who couldn't nixos-rebuild because the second patch here had changed hashes. Looks like there's some irrelevant version number gitlab is appending to the patch file. Using fetchpatch should prevent this from being an issue in the future if I understand correctly.

Sidenote, while I was testing this out, I found I could download the results of the old expression from https://cache.nixos.org by running nix-build -A networkmanager.patches. (EDIT: IRC user couldn't because they had substituters = http://nixos-arm.dezgeg.me/channel). However it's been pointed out to me that fetchurl is supposed to enable preferLocalBuild, which is supposed to prevent this. Is this a bug in Nix?

Sidenote 2, is there a particular reason this derivation doesn't have enableParallelBuilding = true? It's quite a slow build without that, for my 32 thread machine :P

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Fits CONTRIBUTING.md.