Install `llvm-tools-preview` in code coverage action #355

Job	Run time
Lint and format Ruby	10s
Test setup audit toolchain action	13s
Test setup build and test toolchain action	46s
Test setup check minimal versions toolchain action	35s
Test setup code coverage toolchain action	14s
Test setup lint and format toolchain action	42s
Test setup miri toolchain action	11s
Test setup rustdoc toolchain action	10s
Lint and format text	9s
	3m 10s

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