Fix typo/logic bug in modular plugins.

We want to enable plugins only if the environment variable exists!

CC @yongtang, @kvignesh1420, @vnvo2409.

PiperOrigin-RevId: 366056903
Change-Id: Ia6f0918c97cbd6e064e19a02ce3a7d46f48b2799

tensorflow/core/platform/env.h

@@ -642,7 +642,7 @@ struct Register {
     if (try_modular_filesystems) {
       const char* env_value = getenv("TF_USE_MODULAR_FILESYSTEM");
       string load_plugin = env_value ? absl::AsciiStrToLower(env_value) : "";
-      if (!(load_plugin == "true" || load_plugin == "1")) {
+      if (load_plugin == "true" || load_plugin == "1") {
         // We don't register the static filesystem and wait for SIG IO one
         LOG(WARNING) << "Using modular file system for '" << scheme << "."
                      << " Please switch to tensorflow-io"