Port bug 1580128 - Simplify required env variables for fuzzing interface. r=me

mail/app/nsMailApp.cpp

@@ -204,7 +204,7 @@ static int do_main(int argc, char* argv[], char* envp[]) {
 #endif
 
 #ifdef LIBFUZZER
-  if (getenv("LIBFUZZER"))
+  if (getenv("FUZZER"))
     gBootstrap->XRE_LibFuzzerSetDriver(fuzzer::FuzzerDriver);
 #endif