Run some workflows only on a weekly basis #553

Job	Run time
R-CMD-check / macOS-latest (release)	11m 49s
R-CMD-check / windows-latest (devel)	19m 18s
R-CMD-check / windows-latest (release)	26m 13s
R-CMD-check / windows-latest (oldrel)	28m 43s
R-CMD-check / ubuntu-latest (devel)	20m 19s
R-CMD-check / ubuntu-latest (release)	17m 12s
R-CMD-check / ubuntu-latest (oldrel)	14m 52s
R-CMD-check / ubuntu-latest (oldrel-2)	18m 44s
R-CMD-check / ubuntu-latest (oldrel-3)	19m 28s
R-CMD-check / ubuntu-latest (3.6)	5m 23s
	3h 2m 1s

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