ci: activate automerge for github action digest update #178

Job	Run time
Cross checking x86_64-pc-windows-msvc	4m 52s
Cross checking x86_64-pc-windows-gnu	5m 54s
Cross checking x86_64-apple-darwin	6m 34s
Cross checking aarch64-apple-darwin	5m 10s
Cross checking x86_64-unknown-linux-gnu	1m 44s
Cross checking x86_64-unknown-linux-musl	2m 5s
Cross checking aarch64-unknown-linux-gnu	2m 57s
Cross checking i686-unknown-linux-gnu	2m 45s
Cross checking x86_64-unknown-netbsd	3m 3s
Cross checking armv7-unknown-linux-gnueabihf	2m 50s
Result (Cross-CI)	1s
	37m 55s

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