ci: temporarily, run miri only for multiboot2 #953

Job	Run time
build (nightly) / rust	19s
build (stable) / rust	13s
build (msrv) / rust	19s
build no_std (stable) / rust	16s
build no_std (stable) [w/o builder] / rust	13s
build no_std (stable) [Windows] / rust	43s
style (stable) / rust	12s
style (msrv) / rust	24s
build no_std (msrv) / rust	26s
build no_std (nightly) / rust	20s
tests with miri (nightly) / rust	1m 35s
style (nightly) / rust	20s
	5m 20s

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