[CI] Import the DCGM exporter via enroot registry syntax / 用 enroot registry 语法导入 DCGM exporter (#2735)

* fix(ci): import the DCGM exporter via enroot registry syntax

* fix(ci): keep the plain exporter ref where enroot_uri_for_image converts

* test: isolate exporter import cache paths

中文：将 exporter 导入测试的缓存限制在临时目录内。