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/nix-instantiate/nix-instantiate.cc

@@ -168,7 +168,7 @@ static int main_nix_instantiate(int argc, char * * argv)
             for (auto & i : files) {
                 auto p = state->findFile(i);
                 if (auto fn = p.getPhysicalPath())
-                    std::cout << fn->native() << std::endl;
+                    std::cout << fn->string() << std::endl;
                 else
                     throw Error("'%s' has no physical path", p);
             }