x509-cert: use `SubjectPublicKeyInfoOwned::from_key` (#1271) #2042

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	4m 9s
test (1.65.0)	4m 48s
build (1.65.0, wasm32-unknown-unknown)	4m 2s
test (stable)	22s
build (stable, thumbv7em-none-eabi)	17s
build (stable, wasm32-unknown-unknown)	17s
minimal-versions / minimal-versions	38s
	14m 33s

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