fix(core): guard _dupenv_s with _MSC_VER rather than _WIN32

_dupenv_s is an MSVC CRT extension. MinGW-w64 targets Windows, so it
satisfies defined(_WIN32) and selects a branch it cannot link: a CNA
D3D11 or D3D12 cross-build fails with "undefined reference to
`__imp__dupenv_s'", and with it no Windows cross-build of CnaTests links
at all. getenv is the correct fallback for that toolchain and is already
what the non-Windows branch uses.

Author: robertvokac

modules/core/src/System/Environment.cpp

@@ -74,7 +74,10 @@ namespace {
         // getenv("") is unspecified by POSIX. Real .NET returns null for an empty name, which
         // this runtime represents as an unsuccessful lookup and an empty public return value.
         if (name.empty()) return false;
-#if defined(_WIN32)
+// _dupenv_s is an MSVC CRT extension. MinGW-w64 targets Windows but does not export it, so
+// `defined(_WIN32)` selects a branch that cannot link there -- a CNA D3D11 cross-build fails with
+// "undefined reference to `__imp__dupenv_s'". getenv is the correct fallback for that toolchain.
+#if defined(_MSC_VER)
         char* rawValue = nullptr;
         std::size_t valueLength = 0;
         if (_dupenv_s(&rawValue, &valueLength, name.c_str()) != 0 || rawValue == nullptr) {