ci: cancel previous runs when new pushes were made #944

Job	Run time
build (msrv) / rust	18s
build (stable) / rust	20s
build (nightly) / rust	24s
style (msrv) / rust	20s
build no_std (msrv) / rust	27s
build no_std (stable) [w/o builder] / rust	35s
build no_std (stable) / rust	25s
build no_std (stable) [Windows] / rust	1m 0s
style (stable) / rust	21s
build no_std (nightly) / rust	20s
style (nightly) / rust	22s
tests with miri (nightly) / rust	54s
	5m 46s

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