ci: add missing checkout step in purge cache workflow #638

Job	Run time
ci-data	5s
Compile native gem for arm64-darwin	6m 4s
Compile native gem for aarch64-linux	1m 31s
Compile native gem for aarch64-linux-musl	1m 35s
Compile native gem for x86_64-darwin	1m 47s
Compile native gem for x86_64-linux	5m 12s
Compile native gem for x86_64-linux-musl	1m 43s
Compile native gem for x64-mingw-ucrt	1m 40s
	19m 37s

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