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

.woodpecker/e2e.yml

@@ -26,7 +26,7 @@ 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:
       # 标了 Category=Slow 的端到端测试，见 ci/verify-backend.sh
       - bash ci/verify-backend.sh slow