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/libutil/windows/environment-variables.cc

@@ -0,0 +1,17 @@
+#include "environment-variables.hh"
+
+#include "processenv.h"
+
+namespace nix {
+
+int unsetenv(const char *name)
+{
+    return -SetEnvironmentVariableA(name, nullptr);
+}
+
+int setEnv(const char * name, const char * value)
+{
+    return -SetEnvironmentVariableA(name, value);
+}
+
+}