ci: bake cached toolchain into Podman agent and fix nested Buildah [CI]

ci/verify-backend.sh

@@ -73,9 +73,13 @@ fi
 
 # 测试在缓存目录里跑完整的解决方案（不再重复编译）
 log="$cache/last-test.log"
-quic_runtime="$cache/quic-runtime"
-bash "$root/ci/fetch-quic-test-runtime.sh" "$quic_runtime"
-export LD_LIBRARY_PATH="$quic_runtime/runtime${LD_LIBRARY_PATH:+:$LD_LIBRARY_PATH}"
+if [[ -n "${MOSHIRO_QUIC_TEST_RUNTIME:-}" ]]; then
+  quic_runtime="$MOSHIRO_QUIC_TEST_RUNTIME"
+else
+  quic_runtime="$cache/quic-runtime/runtime"
+  bash "$root/ci/fetch-quic-test-runtime.sh" "$cache/quic-runtime"
+fi
+export LD_LIBRARY_PATH="$quic_runtime${LD_LIBRARY_PATH:+:$LD_LIBRARY_PATH}"
 status=0
 # Each test module can spawn its own testhost. Run the whole command in a new
 # session and time out its process group, so orphaned testhosts cannot keep the