ci: temporarily, run miri only for multiboot2 #951

Job	Run time
build (msrv) / rust	19s
build (nightly) / rust	17s
build (stable) / rust	17s
build no_std (stable) [Windows] / rust	44s
style (stable) / rust	14s
build no_std (stable) / rust	19s
build no_std (stable) [w/o builder] / rust	27s
build no_std (nightly) / rust	20s
style (nightly) / rust	16s
tests with miri (nightly) / rust	1m 23s
style (msrv) / rust	19s
build no_std (msrv) / rust	21s
	5m 16s

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