[win32] Set proper NULLCMD in rbconfig.rb [ci skip]

Author: nobu

win32/Makefile.sub

@@ -976,7 +976,7 @@ s,@target_alias@,$(ARCH)-$(PLATFORM),;t t
 s,@target_cpu@,$(ARCH),;t t
 s,@target_vendor@,pc,;t t
 s,@target_os@,$(PLATFORM),;t t
-s,@NULLCMD@,:,;t t
+s,@NULLCMD@,$(NULLCMD),;t t
 s,@CC@,$(CC),;t t
 s,@CPP@,$(CPP),;t t
 s,@CXX@,$$(CC),;t t