scripts/dl_github_archive: use tar -I for ZSTD

scripts/dl_github_archive: use tar -I for ZSTD

Instead of relying on env variables for setting the ZSTD compression
configuration we can simply do what we do for IB, SDK and the rest and
use tar -I to pass "zstd -T0 --ultra -20" directly.

This makes it rather clear what is being done and allows to drop the
zstd CLI max level override as its usually capped at level 19.

Signed-off-by: Robert Marko <robimarko@gmail.com>