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

.woodpecker/verify.yml

@@ -21,7 +21,7 @@ steps:
     when:
       - evaluate: 'not (CI_COMMIT_MESSAGE contains "[SKIP TESTS]")'
     environment:
-      NUGET_PACKAGES: /home/justme/.nuget/packages
+      NUGET_PACKAGES: /home/justme/woodpecker/cache/nuget/packages
     commands:
       # 增量编译 + 快的那批测试（约 20 秒）；慢的端到端测试在 e2e workflow 里和出镜像并行跑。见 ci/verify-backend.sh
       - bash ci/verify-backend.sh fast