pkcs7

x509-cert: use `SubjectPublicKeyInfoOwned::from_key` #2362

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	4m 7s
build (1.65.0, wasm32-unknown-unknown)	4m 6s
build (stable, thumbv7em-none-eabi)	23s
build (stable, wasm32-unknown-unknown)	27s
test (1.65.0)	4m 23s
test (stable)	27s
minimal-versions / minimal-versions	36s
	14m 29s

Provide feedback