Build a minimized Nix with MinGW

At this point many features are stripped out, but this works:

- Can run libnix{util,store,expr} unit tests
- Can run some Nix commands

Co-Authored-By volth <volth@volth.com>
Co-Authored-By Brian McKenna <brian@brianmckenna.org>

src/libexpr/eval.hh

@@ -161,6 +161,8 @@ struct DebugTrace {
     bool isError;
 };
 
+// Don't want Windows function
+#undef SearchPath
 
 class EvalState : public std::enable_shared_from_this<EvalState>
 {