w32: Clarify the environment block encoding.

* src/spawn-w32.c (_gpgrt_spawn_actions_set_envvars): It's an ASCII
string.

In future, we can extend it as UTF-8 string. When doing so, we will
detect ASCII or utf-8, and if needed, convert it to Unicode wchar
string with four-byte-zero terminated, and let the flag have
CREATE_UNICODE_ENVIRONMENT.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>