spki: borrow the public key in `spki::from_key` #842

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	1m 38s
build (1.65.0, wasm32-unknown-unknown)	1m 38s
build (stable, thumbv7em-none-eabi)	24s
build (stable, wasm32-unknown-unknown)	23s
test (1.65.0)	1m 44s
test (stable)	31s
minimal-versions / minimal-versions	38s
	6m 56s

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