yosys: set YOSYS_PLUGIN_PATH

This allows `plugin -i slang.so` (and similar) to work in TCL scripts
without requiring an absolute path to the plugin.

Signed-off-by: Emin <me@emin.chat>

pkgs/by-name/yo/yosys/package.nix

@@ -50,6 +50,7 @@ let
       postBuild = ''
         wrapProgram $out/bin/yosys \
           --set YOSYS_PATH $out/share/yosys \
+          --set YOSYS_PLUGIN_PATH ${pluginPath} \
           ${module_flags}
       '';
       meta.mainProgram = "yosys";