Merge pull request #7976 from rubygems/deivid-rodriguez/gem-command-usable-outside-of-ruby-core

Make ruby-core check not depend on ENV

(cherry picked from commit 6965950)

Author: deivid-rodriguez

bundler/spec/support/env.rb

@@ -3,7 +3,7 @@
 module Spec
   module Env
     def ruby_core?
-      !ENV["GEM_COMMAND"].nil?
+      File.exist?(File.expand_path("../../../lib/bundler/bundler.gemspec", __dir__))
     end
   end
 end