Add musl build action #14

Job	Run time
Build macOS universal binary	2m 36s
Build x86_64-unknown-linux-gnu	2m 41s
Build armv7-unknown-linux-gnueabihf	2m 37s
Build i686-unknown-linux-gnu	2m 43s
Build aarch64-unknown-linux-musl	25s
Build i686-pc-windows-msvc	3m 27s
Build x86_64-pc-windows-msvc	3m 5s
Build x86_64-unknown-linux-musl	23s
Build aarch64-unknown-linux-gnu	2m 36s
	20m 33s

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