Fix ingestion of zstd-compressed tarballs

[bedroge]

zstd -c behaves different than gunzip -c, apparently:

$ gunzip -c ./test.tar.gz > test
$ file test
test: POSIX tar archive (GNU)

$ zstd -c test.tar.zst > test
$  file test
test: Zstandard compressed data (v0.8+), Dictionary ID: None

$ zstd -c -d test.tar.zst > test
$ file test
test: POSIX tar archive (GNU)

[ocaisa]

LGTM