mruby-bin-config: register installer in products instead of bins

`build.bins` is for compiled binaries with bare names; entries are
fed through `exefile()` in `tasks/bin.rake`, which appends
`exts.executable` (e.g. `.exe`) when no extension is present.

When `ENV['OS']` is not `Windows_NT` but the toolchain has
`exts.executable=".exe"` (e.g. cross builds, or environments where
`OS` is unset under visualcpp), `mruby-bin-config` ended up with a
file task at `.../mruby-config` while rake asked for
`.../mruby-config.exe`, aborting with "Don't know how to build task".

Skip the `bins` path and call `build.define_installer` directly with
`mruby_config_path`, mirroring the existing `iscross` branch. The
file task path now always matches the script content's extension
(`.bat` on Windows, no extension elsewhere).

close #6807, reported by UENO, M. (@eunos-1128).

Co-authored-by: Claude <noreply@anthropic.com>

mrbgems/mruby-bin-config/mrbgem.rake

@@ -28,7 +28,7 @@ MRuby::Gem::Specification.new('mruby-bin-config') do |spec|
   if iscross
     build.products << mruby_config_path
   else
-    build.bins << mruby_config
+    build.products << build.define_installer(mruby_config_path)
   end
 
   directory mruby_config_dir