Motivation for this change

  trying https://www.dbvis.com/product_download/dbvis-9.5.7/media/dbvis_unix_9_5_7.tar.gz
    % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                   Dload  Upload   Total   Spent    Left  Speed
    0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
  curl: (22) The requested URL returned error: 404
  error: cannot download dbvis_unix_9_5_7.tar.gz from any mirror

Source has likely been broken since 2017
It's unfree, so we don't have hydra to view breakage history.

Looking at their download options, it seems they only keep the latest major.minor version. 9.5.8 would be the latest in the 9.5 series, and that patch version was released in 2017

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• 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 nixpkgs-review --run "nixpkgs-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)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.