Fix test: set BUNDLE_GEMFILE to bundled_app path so Bundler.root resolves correctly

The test was failing because BUNDLE_GEMFILE was set to a relative "Gemfile"
path (inherited from the parent describe block), causing Bundler.root to
resolve to the process's current working directory instead of bundled_app.
This meant bin_path.directory? could not find the test's bin/ directory.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: masak1yu

spec/bundler/shared_helpers_spec.rb

@@ -347,6 +347,7 @@
       before do
         Dir.mkdir bundled_app(".bundle")
         Dir.mkdir(bundled_app("bin")) unless File.directory?(bundled_app("bin"))
+        ENV["BUNDLE_GEMFILE"] = bundled_app_gemfile.to_s
         ENV["PATH"] = "/usr/bin"
       end
 
@@ -364,6 +365,7 @@
       before do
         Dir.mkdir bundled_app(".bundle")
         FileUtils.rm_rf(bundled_app("bin"))
+        ENV["BUNDLE_GEMFILE"] = bundled_app_gemfile.to_s
         ENV["PATH"] = "/usr/bin"
       end