x509-cert: use `SubjectPublicKeyInfoOwned::from_key` (#1271) #1367

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	4m 16s
fuzz	2m 9s
build (1.65.0, wasm32-unknown-unknown)	4m 50s
test (1.65.0)	14m 0s
build (stable, thumbv7em-none-eabi)	36s
test (stable)	9m 3s
build (stable, wasm32-unknown-unknown)	36s
minimal-versions / minimal-versions	7m 1s
	42m 31s

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