[Concurrency] Don't use getenv() unless SWIFT_STDLIB_HAS_ENVIRON is set.

We don't necessarily have environment variables, which is why we have
SWIFT_STDLIB_HAS_ENVIRON.  Calling getenv() outside of a check of this
means Concurrency won't work in environments where getenv() cannot
be used.

rdar://91050064

Author: al45tair

stdlib/public/Concurrency/Actor.cpp

@@ -332,13 +332,15 @@ static bool swift_task_isCurrentExecutorImpl(ExecutorRef executor) {
 static unsigned unexpectedExecutorLogLevel = 1;
 
 static void checkUnexpectedExecutorLogLevel(void *context) {
+#if SWIFT_STDLIB_HAS_ENVIRON
   const char *levelStr = getenv("SWIFT_UNEXPECTED_EXECUTOR_LOG_LEVEL");
   if (!levelStr)
     return;
 
   long level = strtol(levelStr, nullptr, 0);
   if (level >= 0 && level < 3)
     unexpectedExecutorLogLevel = level;
+#endif // SWIFT_STDLIB_HAS_ENVIRON
 }
 
 SWIFT_CC(swift)