x509-cert: use `SubjectPublicKeyInfoOwned::from_key` #398

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	4m 5s
build (1.65.0, wasm32-unknown-unknown)	4m 27s
build (stable, thumbv7em-none-eabi)	27s
test (1.65.0)	4m 14s
test (stable)	25s
build (stable, wasm32-unknown-unknown)	26s
minimal-versions / minimal-versions	31s
	14m 35s

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