CI testing for UR_L0_CMD_BUFFER_USE_IMMEDIATE_APPEND_PATH

[EwanC]

Merged PR #1975 added a new code path to the L0 command-buffer adapter when a device supports the zeCommandListImmediateAppendCommandListsExp experimental extension. Additional user control of code path selection is provided via the UR_L0_CMD_BUFFER_USE_IMMEDIATE_APPEND_PATH environment variable, which can force the path to be taken or not by the user.

As discussed in PR review comment thread #1975 (comment) we should have CI testing of both code paths enforced through use of the environment variable, as the default code path taken will vary depending on device/L0-driver.