Update install instr. #657

Job	Run time
cs-stan / Coding Standard & Static Analysis	30s
testsuite / testsuite-linux (8.1, lowest, false)	32s
testsuite / testsuite-linux (8.3, highest, false)	48s
testsuite / testsuite-linux (8.4, highest, false)	1m 19s
testsuite / testsuite-linux (8.1, highest, false)	33s
testsuite / testsuite-linux (8.2, highest, false)	34s
	4m 16s

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