Use linux.12xlarge.ephemeral for conda docker builds (#1570)

As `ubuntu.20.04` often OOM/failed to fetch data from RHEL repo

Author: malfet

.github/workflows/build-conda-images.yml

@@ -28,7 +28,7 @@ env:
 
 jobs:
   build-docker:
-    runs-on: ubuntu-22.04
+    runs-on: linux.12xlarge.ephemeral
     strategy:
       matrix:
         cuda_version: ["11.8", "12.1", "cpu"]