spki: borrow the public key in `spki::from_key` #468

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	1m 45s
build (1.65.0, wasm32-unknown-unknown)	2m 14s
build (stable, thumbv7em-none-eabi)	23s
build (stable, wasm32-unknown-unknown)	28s
test (1.65.0)	1m 52s
test (stable)	23s
minimal-versions / minimal-versions	31s
	7m 36s

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