[rubygems/rubygems] Fix generated standalone script for default gems

The installer is actually rewriting the spec's full gem path to be the
one of the newly installed gem, however the accessor was not properly
working for `StubSpecification` instances, and default gems are always
of this type, because they are always present locally.

Fix the accessor to properly update the underlying full specification.

ruby/rubygems@efa41babfa

Author: deivid-rodriguez

lib/bundler/stub_specification.rb

@@ -67,6 +67,10 @@ def full_gem_path
       stub.full_gem_path
     end
 
+    def full_gem_path=(path)
+      stub.full_gem_path = path
+    end
+
     def full_require_paths
       stub.full_require_paths
     end

spec/bundler/install/gems/standalone_spec.rb

@@ -147,9 +147,16 @@
       bundle "lock", :dir => cwd, :artifice => "compact_index"
     end
 
-    it "works" do
+    it "works and points to the vendored copies, not to the default copies" do
       bundle "config set --local path #{bundled_app("bundle")}"
       bundle :install, :standalone => true, :dir => cwd, :artifice => "compact_index", :env => { "BUNDLER_GEM_DEFAULT_DIR" => system_gem_path.to_s }
+
+      load_path_lines = bundled_app("bundle/bundler/setup.rb").read.split("\n").select {|line| line.start_with?("$:.unshift") }
+
+      expect(load_path_lines).to eq [
+        '$:.unshift File.expand_path("#{__dir__}/../#{RUBY_ENGINE}/#{RbConfig::CONFIG["ruby_version"]}/gems/bar-1.0.0/lib")',
+        '$:.unshift File.expand_path("#{__dir__}/../#{RUBY_ENGINE}/#{RbConfig::CONFIG["ruby_version"]}/gems/foo-1.0.0/lib")',
+      ]
     end
   end