ci: bump actions/checkout from 6 to 7 (#1498) #466

Job	Run time
test-miri (s390x-unknown-linux-gnu)	1m 5s
build (stable, wasm32-unknown-unknown)	16s
build (1.85.0, wasm32-unknown-unknown)	15s
test (1.85.0)	1m 31s
build (stable, thumbv7em-none-eabi)	13s
test (stable)	1m 40s
build (1.85.0, thumbv7em-none-eabi)	14s
test-miri (x86_64-unknown-linux-gnu)	52s
minimal-versions / minimal-versions	23s
test-careful	1m 19s
	7m 48s

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