Fix OpenBSD-only typo (nil instead of nullptr)

Author: grynspan

Sources/_TestingInternals/ExecutablePath.cpp

@@ -24,7 +24,7 @@ static constinit std::atomic<const char *> earlyCWD { nullptr };
 /// path (which can occur when manually invoking the test executable.)
 __attribute__((__constructor__(101), __used__))
 static void captureEarlyCWD(void) {
-  if (auto cwd = getcwd(nil, 0)) {
+  if (auto cwd = getcwd(nullptr, 0)) {
     earlyCWD.store(cwd);
   }
 }