spawn:w32: Fix preparing the environment block.

* src/spawn-w32.c (prepare_env_block): Copy environment
variables correctly by memmove.

With "export GNUPGHOME=something", t-spawn.exe failed before the fix.

• GnuPG-bug-id: T8052
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>