librsvg: fix thumbnailer path

[jtojnar]

Motivation for this change

librsvg hooks itself into gdk-pixbuf and then uses gdk-pixbuf-thumbnailer as the thumbnailer, extending its supported MIME type list.

Unfortunately, librsvg assumes the thumbnailer will be located in the same bindir as librsvg binaries would, which is not true on Nix-powered systems.

This commit corrects the bindir path of the thumbnailer to the gdk_pixbuf derivation.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • Linux
• 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/)
• Fits CONTRIBUTING.md.

---

cc @vcunat

[jtojnar]

Confirmed working with Nautilus.

[vcunat]

Not a mass rebuild, most likely

Estimating rebuild amount by counting changed Hydra jobs.                                                                                        
     63 x86_64-darwin                                                                                                                            
    587 x86_64-linux

[jtojnar]

It is a dependency of wrapGAppsHook so I thought it would be mass enough. Did not know about maintainers/scripts/rebuild-amount.sh. What number is considered a mass rebuild?

[vcunat]

There's no fixed number; I'm just trying to choose reasonably to avoid overloading Hydra while not delaying small rebuilds unnecessarily. The "trunk jobset" currently shows ~40k jobs, composed of three platforms, and this will make under a thousand, most likely.