Fix nested bundle exec's when bundler is a default gem

lib/bundler/shared_helpers.rb

@@ -287,9 +287,15 @@ def set_env(key, value)
     public :set_env
 
     def set_bundle_variables
+      # bundler exe & lib folders have same root folder, typical gem installation
       exe_file = File.expand_path("../../../exe/bundle", __FILE__)
-      # for Ruby core repository
-      exe_file = File.expand_path("../../../../bin/bundle", __FILE__) unless File.exist?(exe_file)
+
+      # for Ruby core repository testing
+      exe_file = File.expand_path("../../../bin/bundle", __FILE__) unless File.exist?(exe_file)
+
+      # bundler is a default gem, exe path is separate
+      exe_file = Bundler.rubygems.bin_path("bundler", "bundle", VERSION) unless File.exist?(exe_file)
+
       Bundler::SharedHelpers.set_env "BUNDLE_BIN_PATH", exe_file
       Bundler::SharedHelpers.set_env "BUNDLE_GEMFILE", find_gemfile.to_s
       Bundler::SharedHelpers.set_env "BUNDLER_VERSION", Bundler::VERSION

spec/bundler/shared_helpers_spec.rb

@@ -402,8 +402,10 @@
 
       it "sets BUNDLE_BIN_PATH to the bundle executable file" do
         subject.set_bundle_environment
-        bundle_exe = ruby_core? ? "../../../../../bin/bundle" : "../../../exe/bundle"
-        expect(ENV["BUNDLE_BIN_PATH"]).to eq(File.expand_path(bundle_exe, __FILE__))
+        bundle_exe = ruby_core? ? "../../../bin/bundle" : "../../../exe/bundle"
+        bin_path = ENV["BUNDLE_BIN_PATH"]
+        expect(bin_path).to eq(File.expand_path(bundle_exe, __FILE__))
+        expect(File.exist?(bin_path)).to be true
       end
     end