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

[ASSI]

Jon Turney via Cygwin-apps writes:
> Do you have an example of a cygport which fails without this fix, so I
> can add a test for this?

Anything with a SRC_URI or PATCH_URI that has .zst suffix but not
.tar.zst should suffice.


Regards,
Achim.
-- 
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