Change directory to the path specified by -C option.

The directory specified by `-C` option must be reflected, while the
already loaded spec file must be used, wherever it came from.

Fixes #3953

lib/rubygems/commands/build_command.rb

@@ -85,8 +85,7 @@ def build_gem(gem_name)
       spec = Gem::Specification.load(gemspec)
 
       if options[:build_path]
-        Dir.chdir(File.dirname(gemspec)) do
-          spec = Gem::Specification.load(File.basename(gemspec))
+        Dir.chdir(options[:build_path]) do
           build_package(spec)
         end
       else