Add an environment variable to enable GPU collective cancelling.
When an XLA PjRt GPU client is constructed with `abort_collectives_on_failure`
set to true, it aborts ongoing collectives when machines participating in the
collectives fail. This CL adds an
`XLA_PYTHON_CLIENT_ABORT_COLLECTIVES_ON_FAILURE` environment variable to enable
the behavior.
PiperOrigin-RevId: 776637541