[Runtime] Fix getenv return type in EnvironmentVariables.cpp

Follow-up fix to 4b3a197

Author: kateinoigakukun

stdlib/public/runtime/EnvironmentVariables.cpp

@@ -247,7 +247,7 @@ void swift::runtime::environment::initialize(void *context) {
   // everywhere.
 #define VARIABLE(name, type, defaultValue, help)                               \
   do {                                                                         \
-    const char name##_string = getenv(#name);                                  \
+    const char *name##_string = getenv(#name);                                 \
     if (name##_string)                                                         \
       name##_isSet_variable = true;                                            \
     name##_variable = parse_##type(#name, name##_string, defaultValue);        \