Gitlab CI: switch 'pthread' pocl device to 'cpu'

.gitlab-ci.yml

 Python 3 POCL:
   script: |
-    export PYOPENCL_TEST=portable:pthread
+    export PYOPENCL_TEST=portable:cpu
     export EXTRA_INSTALL="jax[cpu]"
     curl -L -O https://gitlab.tiker.net/inducer/ci-support/raw/main/build-and-test-py-project.sh
     . ./build-and-test-py-project.sh
@@ -52,7 +52,7 @@ Python 3 POCL Nvidia Titan V:
 Python 3 POCL Examples:
   script:
   - test -n "$SKIP_EXAMPLES" && exit
-  - export PYOPENCL_TEST=portable:pthread
+  - export PYOPENCL_TEST=portable:cpu
   - curl -L -O https://gitlab.tiker.net/inducer/ci-support/raw/main/build-py-project-and-run-examples.sh
   - ". ./build-py-project-and-run-examples.sh"
   tags: