[ruby/rubygems] Fix VCR cassette path recording to use git_root

used_cassettes.txt recorded paths relative to source_root (bundler/)
but the Rakefile's check_unused_cassettes task globs relative to the
repo root. Use git_root for consistent path resolution.

ruby/rubygems@92ec2b3ed5

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

Author: hsbt

spec/bundler/support/artifice/vcr.rb

@@ -24,7 +24,7 @@ def handle_request
       end
 
       File.open(USED_CASSETTES_PATH, "a+") do |f|
-        f.puts request_pair_paths.map {|path| Pathname.new(path).relative_path_from(Spec::Path.source_root).to_s }.join("\n")
+        f.puts request_pair_paths.map {|path| Pathname.new(path).relative_path_from(Spec::Path.git_root).to_s }.join("\n")
       end
 
       if recorded_response?