Implement CI for hifive unmatched #232

Job	Run time
test-runtime-functionality	17s
test-runtime-format	11s
build (generic, 32)	30m 40s
build (generic, 64)	28m 50s
build (hifive_unmatched, 64)	26m 19s
build (cva6, 32)	30m 30s
build (cva6, 64)	27m 3s
build (mpfs, 64)	34m 48s
combine-caches	13m 57s
test-system-functionality / test-system (generic, 32)	1m 41s
test-runtime-build / build-runtime (generic, 32)	28s
test-system-functionality / test-system (generic, 64)	1m 41s
test-runtime-build / build-runtime (generic, 64)	27s
test-system-functionality / test-system (mpfs, 64)	6m 18s
test-runtime-build / build-runtime (hifive_unmatched, 64)	51s
test-system-functionality / test-system (cva6, 64)	15m 37s
test-runtime-build / build-runtime (cva6, 32)	26s
test-system-functionality / test-system (hifive_unmatched, 64)	8m 54s
test-runtime-build / build-runtime (cva6, 64)	27s
test-runtime-build / build-runtime (mpfs, 64)	36s
	3h 50m 1s

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