fix(tar) properly extract relative symlinks

symlinks like

  sbin/halt -> ../bin/busybox

were mistakenly dropped before.

Co-authored-by: Andrew Fasano <fasano@mit.edu>

Author: e3krisztian

unblob/handlers/archive/_safe_tarfile.py

@@ -83,10 +83,9 @@ def extract(self, tarinfo: tarfile.TarInfo, extract_root: Path):  # noqa: C901
                     "Converted to extraction relative path.",
                 )
                 tarinfo.linkname = f"./{tarinfo.linkname}"
-            if not is_safe_path(
-                basedir=extract_root,
-                path=extract_root / tarinfo.linkname,
-            ):
+
+            resolved_path = (extract_root / tarinfo.name).parent / tarinfo.linkname
+            if not is_safe_path(basedir=extract_root, path=resolved_path):
                 self.record_problem(
                     tarinfo,
                     "Traversal attempt through link path.",