x509-cert: use `SubjectPublicKeyInfoOwned::from_key` #1362

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	4m 18s
build (1.65.0, wasm32-unknown-unknown)	4m 44s
build (stable, thumbv7em-none-eabi)	33s
fuzz	2m 13s
test (1.65.0)	14m 20s
build (stable, wasm32-unknown-unknown)	35s
test (stable)	9m 9s
minimal-versions / minimal-versions	6m 59s
	42m 51s

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