Add Detail::getEnv wrapper that compiles under UWP

Under UWP it will always return nullptr, so UWP will essentially
behave as if the environment was just empty.

src/catch2/internal/catch_getenv.cpp

@@ -0,0 +1,36 @@
+
+//              Copyright Catch2 Authors
+// Distributed under the Boost Software License, Version 1.0.
+//   (See accompanying file LICENSE_1_0.txt or copy at
+//        https://www.boost.org/LICENSE_1_0.txt)
+
+// SPDX-License-Identifier: BSL-1.0
+
+#include <catch2/internal/catch_getenv.hpp>
+#include <catch2/internal/catch_platform.hpp>
+
+#include <cstdlib>
+
+namespace Catch {
+    namespace Detail {
+
+#if defined( CATCH_PLATFORM_WINDOWS_UWP )
+        char const* getEnv( char const* ) { return nullptr; }
+#else
+
+        char const* getEnv( char const* varName ) {
+#    if defined( _MSC_VER )
+#        pragma warning( push )
+#        pragma warning( disable : 4996 ) // use getenv_s instead of getenv
+#    endif
+
+            return std::getenv( varName );
+
+#    if defined( _MSC_VER )
+#        pragma warning( pop )
+#    endif
+#endif
+    }
+
+} // namespace Detail
+} // namespace Catch