[ci] fix GPG key download for R Linux jobs (fixes #6038) #7178

Job	Run time
r-package (ubuntu-latest, gcc, R 3.6, cmake)	13m 22s
r-package (ubuntu-latest, gcc, R 4.2, cmake)	13m 42s
r-package (ubuntu-latest, clang, R 3.6, cmake)	11m 32s
r-package (ubuntu-latest, clang, R 4.2, cmake)	15m 38s
r-package (macOS-latest, gcc, R 4.2, cmake)	16m 16s
r-package (macOS-latest, clang, R 4.2, cmake)	19m 16s
r-package (windows-latest, MINGW, R 3.6, cmake)	16m 22s
r-package (windows-latest, MINGW, R 4.2, cmake)	20m 26s
r-package (windows-2019, MSVC, R 3.6, cmake)	15m 23s
r-package (windows-2022, MSVC, R 4.2, cmake)	15m 16s
r-package (windows-latest, MINGW, R 3.6, cran)	23m 9s
r-package (windows-latest, MINGW, R 4.2, cran)	19m 17s
r-package (ubuntu-latest, gcc, R 4.2, cran)	20m 1s
r-package (macOS-latest, clang, R 4.2, cran)	25m 35s
r-rchk (ubuntu-latest, gcc, R 4.2, cran)	12m 57s
r-sanitizers (ubuntu-latest, R-devel, gcc ASAN/UBSAN)	21m 55s
r-sanitizers (ubuntu-latest, R-devel, clang ASAN/UBSAN)	31m 21s
r-package (debian, R-devel, clang)	20m 50s
all-r-package-jobs-successful	2s
	5h 32m 20s

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