zathura: fix fish completions

The fish completion checks the plugin directory for supported file types
to complete. However the plugin dir checked was the one of the
zathura_core derivation which of course has no plugin dir. We now patch
up the referenced path in the wrapper derivation.

pkgs/applications/misc/zathura/wrapper.nix

@@ -7,10 +7,19 @@ symlinkJoin {
 
   buildInputs = [ makeWrapper ];
 
-  postBuild = ''
+  postBuild = let
+    fishCompletion = "share/fish/vendor_completions.d/zathura.fish";
+  in ''
     makeWrapper ${zathura_core.bin}/bin/zathura $out/bin/zathura \
       --prefix PATH ":" "${lib.makeBinPath [ file ]}" \
       --add-flags --plugins-dir="$out/lib/zathura"
+
+    # zathura fish completion references the zathura_core derivation to
+    # check for supported plugins which live in the wrapper derivation,
+    # so we need to fix the path to reference $out instead.
+    rm "$out/${fishCompletion}"
+    substitute "${zathura_core.out}/${fishCompletion}" "$out/${fishCompletion}" \
+      --replace "${zathura_core.out}" "$out"
   '';
 
   meta = with lib; {