include: make uv_process_options_t.cwd const

There is no reason for it to be non-const, libuv treats it as a
read-only input.  Make it a pointer to a const string.

include/uv.h

@@ -1417,7 +1417,7 @@ typedef struct uv_process_options_s {
    * If non-null this represents a directory the subprocess should execute
    * in. Stands for current working directory.
    */
-  char* cwd;
+  const char* cwd;
   /*
    * Various flags that control how uv_spawn() behaves. See the definition of
    * `enum uv_process_flags` below.