[ci] ensure correct R version is used on GitHub Actions (fixes #5640) #7378

Job	Run time
r-package (debian, R-devel, clang)	18m 22s
r-sanitizers (ubuntu-latest, R-devel, gcc ASAN/UBSAN)	23m 14s
r-sanitizers (ubuntu-latest, R-devel, clang ASAN/UBSAN)	30m 47s
r-package (ubuntu-latest, gcc, R 3.6, cmake)	14m 46s
r-package (ubuntu-latest, gcc, R 4.3, cmake)	13m 41s
r-package (ubuntu-latest, clang, R 3.6, cmake)	15m 15s
r-package (ubuntu-latest, clang, R 4.3, cmake)	12m 50s
r-package (macOS-latest, gcc, R 4.3, cmake)	16m 28s
r-package (macOS-latest, clang, R 4.3, cmake)	15m 24s
r-package (windows-latest, MINGW, R 3.6, cmake)	18m 16s
r-package (windows-latest, MINGW, R 4.2, cmake)	19m 47s
r-package (windows-2019, MSVC, R 3.6, cmake)	12m 0s
r-package (windows-2022, MSVC, R 4.2, cmake)	16m 41s
r-package (windows-latest, MINGW, R 3.6, cran)	24m 19s
r-package (windows-latest, MINGW, R 4.2, cran)	22m 21s
r-package (ubuntu-latest, gcc, R 4.3, cran)	27m 53s
r-package (macOS-latest, clang, R 4.3, cran)	33m 39s
r-rchk (ubuntu-latest, gcc, R 4.3, cran)	14m 9s
all-r-package-jobs-successful	2s
	5h 49m 54s

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