Fix memory leak

Some [Linux manpages][1] state

> This function makes copies of the strings pointed to by name and
> value (by contrast with putenv(3)).

and the [OpenGroup][2] manpage also states that the strings are
copied, so indeed they can and should be freed afterwards.

[1]: https://man7.org/linux/man-pages/man3/setenv.3.html
[2]: https://pubs.opengroup.org/onlinepubs/009695399/functions/setenv.html

ocamltest/run_unix.c

@@ -147,6 +147,8 @@ static void update_environment(array local_env)
       memcpy(value, pos_eq + 1, value_length);
       value[value_length] = '\0';
       setenv(name, value, 1); /* 1 means overwrite */
+      free(name);
+      free(value);
     }
   }
 }