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

[ASSI]

yak_ex via Cygwin-apps writes:
> `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.

Indeed, sorry for that.

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

Hmm. I'd would have preferred to use the --keep option (which is the
default for unzstd anyway) and save the redirection (also for the other
programs that offer that option).


Regards,
Achim.
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