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/libcmd/common-eval-args.cc

@@ -181,7 +181,7 @@ Bindings * MixEvalArgs::getAutoArgs(EvalState & state)
                 v->mkString(arg.s);
             },
             [&](const AutoArgFile & arg) {
-                v->mkString(readFile(arg.path));
+                v->mkString(readFile(arg.path.string()));
             },
             [&](const AutoArgStdin & arg) {
                 v->mkString(readFile(STDIN_FILENO));