Make the Kernel timer the default clock for Zephyr #2778

Job	Run time
check-diff / check	18s
serialization / serialization-tests	7m 58s
c / benchmarking / benchmark-tests	5m 41s
c / ccpp / regular-tests (ubuntu-latest)	20m 51s
c / arduino / arduino-tests (ubuntu-latest)	3m 4s
c / default / regular-tests (ubuntu-latest)	24m 58s
c / verifier / run (ubuntu-latest)	5m 26s
cpp / cpp-benchmark-tests / benchmark-tests	11m 15s
c / zephyr / zephyr-tests	42m 43s
cpp / cpp-tests / regular-tests (ubuntu-latest)	48m 17s
rs / rs-benchmark-tests / benchmark-tests	15m 37s
cpp / cpp-ros2-tests / cpp-ros2-tests	37m 33s
py / py-tests / regular-tests (ubuntu-latest)	16m 39s
rs / rs-tests / regular-tests (ubuntu-latest, stable)	8m 50s
ts / benchmarking / benchmark-tests	3m 48s
ts / ts-tests / regular-tests (ubuntu-latest)	20m 34s
	4h 33m 32s

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