Suppress pip cache and root-user warnings in Linux container CI jobs

Set PIP_CACHE_DIR=/tmp/pip-cache and PIP_ROOT_USER_ACTION=ignore in the
container env for test-wheel-linux and coverage-linux jobs. These jobs
run as root in Ubuntu containers where /github/home/.cache/pip is not
writable, causing ~54 harmless warnings per CI run.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: leofang

.github/workflows/coverage.yml

@@ -50,6 +50,8 @@ jobs:
       image: ubuntu:22.04
       env:
         NVIDIA_VISIBLE_DEVICES: ${{ env.NVIDIA_VISIBLE_DEVICES }}
+        PIP_CACHE_DIR: "/tmp/pip-cache"
+        PIP_ROOT_USER_ACTION: "ignore"
     steps:
       - name: Ensure GPU is working
         run: nvidia-smi

.github/workflows/test-wheel-linux.yml

@@ -85,6 +85,8 @@ jobs:
       image: ubuntu:22.04
       env:
         NVIDIA_VISIBLE_DEVICES: ${{ env.NVIDIA_VISIBLE_DEVICES }}
+        PIP_CACHE_DIR: "/tmp/pip-cache"
+        PIP_ROOT_USER_ACTION: "ignore"
     steps:
       - name: Ensure GPU is working
         run: nvidia-smi