Use `actions/v1` tag and update instructions #757

Job	Run time
routine / routine	5m 23s
R-CMD-check / setup	27s
website / pkgdown	4m 24s
R-CMD-check / macOS-latest (4.3.1)	8m 19s
R-CMD-check / windows-latest (devel)	10m 28s
R-CMD-check / windows-latest (4.3.1)	15m 55s
R-CMD-check / windows-latest (4.1)	13m 35s
R-CMD-check / windows-latest (3.6)	11m 21s
R-CMD-check / ubuntu-20.04 (devel)	7m 50s
R-CMD-check / ubuntu-20.04 (4.3.1)	4m 38s
R-CMD-check / ubuntu-20.04 (4.2.3)	5m 15s
R-CMD-check / ubuntu-20.04 (4.1.3)	5m 24s
R-CMD-check / ubuntu-20.04 (4.0.5)	5m 17s
R-CMD-check / ubuntu-20.04 (3.6.3)	5m 57s
	1h 44m 13s

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