Use vendor script to vendor lowrisc_ip #22

Job	Run time
riscv-tests-build	19s
lint	14s
simulator-build (pipeline, muntjac_pipeline)	59s
simulator-build (core, muntjac_core)	1m 45s
riscv-tests-run (pipeline, muntjac_pipeline)	16s
riscv-tests-run (core, muntjac_core)	17s
	3m 50s

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