[ci] fix installation of clang-18 (fixes #6306) (#6307) #7873

Job	Run time
r-package (ubuntu-latest, gcc, R 3.6, cmake)	8m 4s
r-package (ubuntu-latest, gcc, R 4.3, cmake)	9m 25s
r-package (ubuntu-latest, clang, R 3.6, cmake)	8m 16s
r-package (ubuntu-latest, clang, R 4.3, cmake)	9m 8s
r-package (macOS-latest, gcc, R 4.3, cmake)	12m 16s
r-package (macOS-latest, clang, R 4.3, cmake)	11m 11s
r-package (windows-latest, MINGW, R 3.6, cmake)	17m 28s
r-package (debian, R-devel, clang-16)	15m 58s
r-package (windows-latest, MINGW, R 4.3, cmake)	23m 18s
r-package (debian, R-devel, clang-17)	16m 4s
r-package (windows-2019, MSVC, R 3.6, cmake)	14m 11s
r-package (debian, R-devel, clang-18)	15m 58s
r-package (windows-2022, MSVC, R 4.3, cmake)	18m 8s
r-package (windows-latest, MINGW, R 3.6, cran)	26m 20s
r-sanitizers (ubuntu-latest, R-devel, gcc ASAN/UBSAN)	14m 29s
r-package (windows-latest, MINGW, R 4.3, cran)	23m 5s
r-sanitizers (ubuntu-latest, R-devel, clang ASAN/UBSAN)	30m 4s
r-package (ubuntu-latest, gcc, R 4.3, cran)	15m 8s
r-package (macOS-latest, clang, R 4.3, cran)	16m 41s
r-rchk (ubuntu-latest, gcc, R 4.3, cran)	9m 17s
all-r-package-jobs-successful	4s
	5h 14m 33s

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