Fix uninstall_gem to set GEM_HOME instead of install_dir

Gem::Uninstaller with install_dir uses File.realpath for @gem_home but
not for the specification_record path lookup, causing path mismatches
on some CI environments. Instead, temporarily set GEM_HOME and call
Gem.clear_paths to mimic the old subprocess-based gem_command behavior.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hsbt

bundler/spec/support/helpers.rb

@@ -346,14 +346,19 @@ def uninstall_gem(name, options = {})
 
       gem_home = options.dig(:env, "GEM_HOME") || system_gem_path.to_s
 
-      uninstaller = Gem::Uninstaller.new(
-        name,
-        install_dir: gem_home,
-        ignore: true,
-        executables: true,
-        all: true
-      )
-      uninstaller.uninstall
+      with_env_vars("GEM_HOME" => gem_home) do
+        Gem.clear_paths
+
+        uninstaller = Gem::Uninstaller.new(
+          name,
+          ignore: true,
+          executables: true,
+          all: true
+        )
+        uninstaller.uninstall
+      ensure
+        Gem.clear_paths
+      end
     end
 
     def installed_gems_list(options = {})