x509-cert: use `SubjectPublicKeyInfoOwned::from_key` (#1271) #1144

Job	Run time
build (1.70.0, thumbv7em-none-eabi)	23s
build (1.70.0, wasm32-unknown-unknown)	28s
test (1.70.0)	1m 3s
build (stable, thumbv7em-none-eabi)	23s
test (stable)	1m 0s
build (stable, wasm32-unknown-unknown)	22s
minimal-versions / minimal-versions	59s
	4m 38s

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