Upgrade GitHub Actions self-hosted runner (#2423)

The self-hosted runner version 2.311.0 is below the new minimum required
version (v2.329.0). GitHub will block connections from runners older
than v2.329.0 starting March 2, 2026.

Update to v2.331.0 (latest stable) to ensure continued CI operation.

Author: rjpower

infra/tpu-ci/vm_manager.py

@@ -227,7 +227,7 @@ def get_startup_script() -> str:
 udevadm trigger --subsystem-match=pci
 
 echo "Installing GitHub Actions runner..."
-RUNNER_VERSION="2.311.0"
+RUNNER_VERSION="2.331.0"
 RUNNER_USER="github-runner"
 
 if ! id -u $RUNNER_USER > /dev/null 2>&1; then