Install gems for rbx platform only when running rbx

or bundle install fails on ruby 3 because rubysl ~> 2.0 doesn't support ruby 3

this follows up 59dbc1246a

gemfiles/active_record_60.gemfile

@@ -16,11 +16,13 @@ platforms :jruby do
   gem 'activerecord-jdbcpostgresql-adapter', require: false
   gem 'activerecord-jdbcmysql-adapter', require: false
 end
-platforms :rbx do
-  gem 'rubysl', '~> 2.0'
-  gem 'racc'
-  gem 'minitest'
-  gem 'rubinius-developer_tools'
+if RUBY_ENGINE == 'rbx'
+  platforms :rbx do
+    gem 'rubysl', '~> 2.0'
+    gem 'racc'
+    gem 'minitest'
+    gem 'rubinius-developer_tools'
+  end
 end
 
 gemspec path: '../'