[build] update rbe gating for ruby tests to use the right toggle

Author: titusfortner

rb/spec/integration/selenium/webdriver/spec_support/test_environment.rb

@@ -142,7 +142,7 @@ def bazel_java
         end
 
         def rbe?
-          Dir.pwd.start_with?('/mnt/engflow')
+          ENV['REMOTE_BUILD'] == '1'
         end
 
         def reset_remote_server