fix(ci): stream Cargo cache snapshots through tar

Export BuildKit cache snapshots as a tar stream and extract directly into the temporary directory to avoid local exporter stalls. Preserve snapshot validation and pipeline error propagation before replacing the saved cache.

Co-authored-by: Codex <noreply@openai.com>

.github/actions/cargo-cache/cargo-cache.sh

@@ -42,8 +42,11 @@ else
   mkdir -p "$(dirname "$directory")"
   temporary="$(mktemp -d "${directory}.XXXXXX")"
   trap 'rm -rf "$temporary"' EXIT
-  mkdir "$temporary/context"
-  "${command[@]}" --output "type=local,dest=$temporary/snapshot" "$temporary/context"
+  mkdir "$temporary"/{context,snapshot}
+  # tar 流避开 local exporter 的逐文件同步阻塞，直接解包以免额外落盘整份归档。
+  # https://github.com/moby/buildkit/issues/2950
+  "${command[@]}" --output type=tar,dest=- "$temporary/context" \
+    | tar -xf - -C "$temporary/snapshot"
   if usable "$temporary/snapshot"; then
     rm -rf "$directory"
     mv "$temporary/snapshot" "$directory"