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

[Yasutaka ATARASHI]

ASSI via Cygwin-apps writes:
> That's true for bunzip2 and gunzip, but not for zstd (where the output
> file can be independently specified).

Ah ha. The attached patch recovers to use the `-o` option for `unzstd`.

Regards,
Yasutaka ATARASHI
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