fix: guard exist? for non-absolute paths in npm packages

Match the cargo extension pattern: only check exist? for absolute
paths so that test mocks with relative Pathname work correctly.

Author: mvanhorn

Library/Homebrew/bundle/extensions/npm.rb

@@ -32,7 +32,8 @@ def packages
           packages = @packages
           return packages if packages
 
-          @packages = if (npm = package_manager_executable) && npm.exist?
+          @packages = if (npm = package_manager_executable) &&
+                         (!npm.to_s.start_with?("/") || npm.exist?)
             parse_package_list(`#{npm} list -g --depth=0 --json 2>/dev/null`)
           end
           return [] if @packages.nil?