Support out-of-place build in ruby/ruby repo

Author: hsbt

spec/bundler/support/builders.rb

@@ -425,7 +425,7 @@ def required_ruby_version=(*reqs)
     end
 
     class BundlerBuilder
-      SPEC = Gem::Specification.load(Spec::Path.relative_gemspec)
+      SPEC = Gem::Specification.load(Spec::Path.source_root.join(Spec::Path.relative_gemspec).to_s)
 
       def initialize(context, name, version)
         raise "can only build bundler" unless name == "bundler"