ci: modify ci.yml to let `cargo fmt/clippy` runs on nightly channel (… #637

Job	Run time
test (x86_64-unknown-linux-gnu, stable)	5m 55s
test (x86_64-unknown-linux-gnu, nightly)	13s
test (i686-unknown-linux-gnu, stable)	1m 53s
test (i686-unknown-linux-gnu, nightly)	16s
test (aarch64-unknown-linux-gnu, stable)	4m 47s
test (aarch64-unknown-linux-gnu, nightly)	17s
test (armv7-unknown-linux-gnueabihf, stable)	2m 24s
test (armv7-unknown-linux-gnueabihf, nightly)	15s
test (riscv64gc-unknown-linux-gnu, stable)	1m 18s
test (riscv64gc-unknown-linux-gnu, nightly)	15s
test (s390x-unknown-linux-gnu, stable)	2m 37s
test (s390x-unknown-linux-gnu, nightly)	16s
test (x86_64-apple-darwin, stable)	1m 11s
test (x86_64-apple-darwin, nightly)	19s
test (aarch64-apple-darwin, stable)	5m 11s
test (aarch64-apple-darwin, nightly)	17s
test (x86_64-pc-windows-gnu, stable)	3m 28s
test (x86_64-pc-windows-gnu, nightly)	48s
test (x86_64-pc-windows-msvc, stable)	6m 19s
test (x86_64-pc-windows-msvc, nightly)	42s
test (i686-pc-windows-gnu, stable)	8m 25s
test (i686-pc-windows-gnu, nightly)	48s
test (i686-pc-windows-msvc, stable)	6m 39s
test (i686-pc-windows-msvc, nightly)	35s
	55m 8s

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