ci: periodically prune old GitHub Actions caches #637

Job	Run time
ci-data	4s
Compile native gem for aarch64-linux	5m 58s
Compile native gem for x86_64-linux	5m 18s
Compile native gem for x64-mingw-ucrt	5m 16s
Compile native gem for arm64-darwin	5m 36s
Compile native gem for x86_64-darwin	5m 19s
Compile native gem for aarch64-linux-musl	6m 9s
Compile native gem for x86_64-linux-musl	5m 26s
	39m 6s

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