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>

scripts/dl_github_archive.py

@@ -139,9 +139,7 @@ class Path(object):
         if ts is not None:
             args.append('--mtime=@%d' % ts)
         if into.endswith('.zst'):
-            envs['ZSTD_CLEVEL'] = '20'
-            envs['ZSTD_NBTHREADS'] = '0'
-            args.append('--zstd')
+            args.append('-I zstd -T0 --ultra -20')
         elif into.endswith('.xz'):
             envs['XZ_OPT'] = '-7e'
             args.append('-J')