test_gem_command_manager.rb: Fix the environment variable

The previous one failed:
https://github.com/ruby/ruby/actions/runs/17504307509/job/49724437021

$GITHUB_ACTION_REPOSITORY points to the owner of the action,
so it might not be ruby/ruby.

Author: k0kubun

test/rubygems/test_gem_command_manager.rb

@@ -72,7 +72,7 @@ def test_find_command_unknown
   end
 
   def test_find_command_unknown_suggestions
-    omit "randomly fails on ruby/ruby CI" if ENV["GITHUB_ACTION_REPOSITORY"] == "ruby/ruby"
+    omit "randomly fails on ruby/ruby CI" if ENV["GITHUB_REPOSITORY"] == "ruby/ruby"
     e = assert_raise Gem::UnknownCommandError do
       @command_manager.find_command "pish"
     end