fish: fix fishConfig test for embedded web_config

share/fish/tools/ is no longer installed, so extract the embedded
web_config files via `status get-file`.

pkgs/by-name/fi/fish/package.nix

@@ -404,10 +404,17 @@ stdenv.mkDerivation (finalAttrs: {
       fishConfig =
         let
           fishScript = writeText "test.fish" ''
-            set -x __fish_bin_dir ${finalAttrs.finalPackage}/bin
-            echo $__fish_bin_dir
-            cp -r ${finalAttrs.finalPackage}/share/fish/tools/web_config/* .
-            chmod -R +w *
+            # webconfig.py locates fish via $fish_bin_dir, which fish_config
+            # normally exports from the read-only $__fish_bin_dir.
+            set -x fish_bin_dir $__fish_bin_dir
+            echo $fish_bin_dir
+
+            # The web_config tool is embedded in the binary, so extract it.
+            for f in (status list-files tools/web_config)
+                mkdir -p (path dirname $f)
+                status get-file $f > $f
+            end
+            cd tools/web_config
 
             # if we don't set `delete=False`, the file will get cleaned up
             # automatically (leading the test to fail because there's no