Bump actions/checkout from 3 to 4 #70

Job	Run time
Check (stable)	21s
(OS X) Builds (aarch64-apple-ios)	5m 46s
Check (beta)	23s
Bench Compile (stable)	1m 35s
Check (1.60.0)	3m 22s
(Linux) Builds (aarch64-linux-android)	7m 0s
(OS X) Builds (x86_64-apple-darwin)	5m 35s
(Linux) Builds (x86_64-unknown-linux-musl)	5m 36s
(Linux) Builds (wasm32-unknown-unknown)	4m 37s
(Windows) Builds (x86_64-pc-windows-msvc)	4m 43s
Rustfmt (nightly)	17s
Test Suite (stable)	2m 16s
Test Suite (1.60.0)	4m 2s
	45m 33s

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