Switch pkgci CPU ONNX tests to use standard GitHub runner.

[ScottTodd]

Follow-up on https://github.com/iree-org/iree/pull/17371#discussion_r1598719094, progress on https://github.com/iree-org/iree/issues/17370.

CPU tests don't need to use a GPU machine so this moves the ONNX op tests to a standard GitHub-hosted runner. The model tests still use the (currently disabled) GPU machine since it has a cache of downloaded model files and enough CPU cores to compile and run the included tests quickly.

ci-exactly: build_packages, regression_test_cpu