x509-cert: use `SubjectPublicKeyInfoOwned::from_key` #394

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	4m 16s
build (1.65.0, wasm32-unknown-unknown)	4m 8s
build (stable, thumbv7em-none-eabi)	23s
build (stable, wasm32-unknown-unknown)	23s
test (1.65.0)	4m 12s
test (stable)	24s
minimal-versions / minimal-versions	34s
	14m 20s

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