use the usual package output directory for build summary

This simplifies things for windows where we are currently outputing the
summary into the sources location instead of the usual output one.
No change is expected on linux where the package_dir is a subdirectory
of the project_root

Author: chouquette

lib/omnibus/project.rb

@@ -1481,7 +1481,7 @@ def store_package_duration(packager, duration)
     end
 
     def write_build_summary
-      out_path = "#{Config.project_root}/pkg/build-summary.json"
+      out_path = "#{Config.package_dir}/build-summary.json"
       log.info(log_key) { "Writing build summary to #{out_path}" }
       File.open(out_path, "w") do |f|
         f.write(FFI_Yajl::Encoder.encode(build_summary.to_hash, pretty: true))