kde5.plasma: 5.7.0 -> 5.7.1

- Update from Plasma 5.7.0 to 5.7.1.
- Remove the version number from the directory storing the Plasma Nix
  expressions. It is not necessary to version the Nix expressions now
  that we keep only one version in Nixpkgs.
- Fix a bug in generate-kde-plasma.sh which prevented it from finding
  its helper script.
- Automatically redirect the output of generate-kde-plasma.sh to make
  the update script even easier to use.