fix: upgrade to setup-r@v2 #63

Job	Run time
(SWC) Shell novice (Linux)	1m 47s
(SWC) Shell novice (macOS)	8m 3s
(SWC) Shell novice (Windows)	4m 23s
(DC) R Intro Geospatial (Linux)	1m 28s
(DC) R Intro Geospatial (macOS)	5m 19s
(DC) R Intro Geospatial (Windows)	5m 19s
(LC) Intro to Git (Linux)	1m 26s
(LC) Intro to Git (macOS)	4m 34s
(LC) Intro to Git (Windows)	4m 36s
(DC) Foundations of Astronomical Data Science (Linux)	1m 32s
(CP) Lesson Example (Linux)	1m 31s
	39m 58s

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