nsd: 4.1.19 -> 4.1.20

Semi-automatic update. These checks were performed:

- built on NixOS
- ran `/nix/store/4za086gx5nlvf44i0syc2y6lprhh49kz-nsd-4.1.20/bin/nsd -h` got 0 exit code
- ran `/nix/store/4za086gx5nlvf44i0syc2y6lprhh49kz-nsd-4.1.20/bin/nsd -v` and found version 4.1.20
- ran `/nix/store/4za086gx5nlvf44i0syc2y6lprhh49kz-nsd-4.1.20/bin/nsd -h` and found version 4.1.20
- ran `/nix/store/4za086gx5nlvf44i0syc2y6lprhh49kz-nsd-4.1.20/bin/nsd-checkzone -h` got 0 exit code
- ran `/nix/store/4za086gx5nlvf44i0syc2y6lprhh49kz-nsd-4.1.20/bin/nsd-checkzone -h` and found version 4.1.20
- found 4.1.20 with grep in /nix/store/4za086gx5nlvf44i0syc2y6lprhh49kz-nsd-4.1.20
- found 4.1.20 in filename of file in /nix/store/4za086gx5nlvf44i0syc2y6lprhh49kz-nsd-4.1.20

cc "@hrdinka"