[PATCH cygport] lib/src_prep.cygpart: Fix incorrect options passed to unzstd in unpack()

[yak_ex]

Hi maintainers,

This is my first post here.
The attached tiny patch fixes incorrect options passed to unzstd in unpack()
in lib/src_prep.cygpart.

`unpack()` invokes `unzstd` for a *.zst file as
`unzstd -qo file.zst`
Then, an error occurs as 'stdin is a console, aborting',
because the `-o` option is to specify an output file name.

There are some choices to fix it. This patch aligns with handling .gz
and .bz2 like
`unzstd -c file.zst > file`.

Regards,
Yasutaka ATARASHI
---
Yasutaka ATARASHI <yak1ex@gmail.com>
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