Fix loadError occurring in nested bundle exec calls

lib/bundler/shared_helpers.rb

@@ -300,7 +300,12 @@ def set_env(key, value)
 
     def set_bundle_variables
       begin
-        Bundler::SharedHelpers.set_env "BUNDLE_BIN_PATH", Bundler.rubygems.bin_path("bundler", "bundle", VERSION)
+        exe_file = Bundler.rubygems.bin_path("bundler", "bundle", VERSION)
+        unless File.exist?(exe_file)
+          exe_file = File.expand_path("../../../exe/bundle", __FILE__)
+        end
+
+        Bundler::SharedHelpers.set_env "BUNDLE_BIN_PATH", exe_file
       rescue Gem::GemNotFoundException
         Bundler::SharedHelpers.set_env "BUNDLE_BIN_PATH", File.expand_path("../../../exe/bundle", __FILE__)
       end

spec/bundler/shared_helpers_spec.rb

@@ -378,6 +378,16 @@
       it_behaves_like "ENV['RUBYLIB'] gets set correctly"
     end
 
+    context "bundle executable in ENV['BUNDLE_BIN_PATH'] does not exist" do
+      before { ENV["BUNDLE_BIN_PATH"] = "/does/not/exist" }
+      before { Bundler.rubygems.replace_bin_path [], [] }
+
+      it "sets BUNDLE_BIN_PATH to the bundle executable file" do
+        subject.set_bundle_environment
+        expect(ENV["BUNDLE_BIN_PATH"]).to eq(File.expand_path("../../../exe/bundle", __FILE__))
+      end
+    end
+
     context "ENV['RUBYLIB'] already contains the bundler's ruby version lib path" do
       let(:ruby_lib_path) { "stubbed_ruby_lib_dir" }