xds: enableSpiffe also checks the new env var per gRFC-A87

Author: rockspore

xds/src/main/java/io/grpc/xds/internal/security/certprovider/FileWatcherCertificateProviderProvider.java

@@ -36,9 +36,10 @@
  */
 public final class FileWatcherCertificateProviderProvider implements CertificateProviderProvider {
 
+  // TODO(lwge): Remove the old env var check once it's confirmed to be unused.
   @VisibleForTesting
   public static boolean enableSpiffe = GrpcUtil.getFlag("GRPC_EXPERIMENTAL_SPIFFE_TRUST_BUNDLE_MAP",
-      false);
+      false) || GrpcUtil.getFlag("GRPC_EXPERIMENTAL_XDS_MTLS_SPIFFE", false);
   private static final String CERT_FILE_KEY = "certificate_file";
   private static final String KEY_FILE_KEY = "private_key_file";
   private static final String ROOT_FILE_KEY = "ca_certificate_file";