Fix cross-compile path nesting and ruby-version-file warning in CD (#156)

* Fix cross-compile path nesting for native extensions

The compile.rake was adding the host Ruby version to ext.lib_dir while
extconf.rb added the target Ruby version, causing double nesting like
lib/code_ownership/4.0/3.2/code_ownership.so instead of the expected
lib/code_ownership/3.2/code_ownership.so.

This broke gem loading on Ruby 4.0 with "cannot load such file" errors.

* Fix ruby-version-file warning in CD workflow

Replace ruby-version-file with explicit ruby-version to avoid
"Unexpected input" warnings from the setup-ruby action.

Author: joemsak

.github/workflows/cd.yml

@@ -43,7 +43,7 @@
 
         - uses: ruby/setup-ruby@v1
           with:
-            ruby-version-file: .ruby-version
+            ruby-version: '4.0'
 
         - uses: oxidize-rb/actions/cross-gem@v1
           id: cross-gem

rakelib/compile.rake

@@ -1,6 +1,5 @@
 require 'rb_sys/extensiontask'
 
 RbSys::ExtensionTask.new('code_ownership', GEMSPEC) do |ext|
-  ruby_minor = RUBY_VERSION[/\d+\.\d+/]
-  ext.lib_dir = "lib/code_ownership/#{ruby_minor}"
+  ext.lib_dir = 'lib/code_ownership'
 end