Fix loading the gemspec file from rake with Ruby 3.3.

Calculating the current directory name from within the gemspec would
fail.

`__FILE__` was '(eval)' with Ruby < 3.3. `File.dirname(__FILE__)` is
'.'.

`__FILE__` is '(eval at /path/Rakefile:linenumber)' with Ruby >= 3.3.
`File.dirname(__FILE__)` is '(eval at /path'.

Rakefile

@@ -11,11 +11,11 @@ rescue LoadError, RuntimeError
 end
 
 BASE_DIR = File.expand_path(File.dirname(__FILE__))
+GEMSPEC_PATH = File.join(BASE_DIR, 'tzinfo.gemspec')
+spec = TOPLEVEL_BINDING.eval(File.read(GEMSPEC_PATH), GEMSPEC_PATH)
 
 task default: [:test]
 
-spec = eval(File.read('tzinfo.gemspec'))
-
 class TZInfoPackageTask < Gem::PackageTask
   alias_method :orig_sh, :sh
   private :orig_sh