ci: temporarily, run miri only for multiboot2 #955

Job	Run time
build (msrv) / rust	19s
build (nightly) / rust	23s
build (stable) / rust	16s
build no_std (stable) [w/o builder] / rust	24s
build no_std (stable) / rust	14s
build no_std (stable) [Windows] / rust	39s
style (stable) / rust	28s
style (msrv) / rust	28s
build no_std (msrv) / rust	21s
style (nightly) / rust	22s
build no_std (nightly) / rust	20s
tests with miri (nightly) / rust	1m 24s
	5m 38s

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