libjpeg-turbo: Fix tests on RISC-V

[zhaofengli]

Motivation for this change

This fixes the tests on RISC-V caused by floating point behavior differences (see linked upstream issue).
This only has an effect on native RISC-V builds (with #115406), but I guess it doesn't hurt to merge it now.

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/)
• Added a release notes entry if the change is major or breaking
• Fits CONTRIBUTING.md.