elliptic-curve: bump `crypto-bigint` to v0.6.0-pre.12 #1784

Job	Run time
minimal-versions	32s
build (1.60.0, thumbv7em-none-eabi)	2m 7s
test (1.60.0)	2m 12s
build (1.60.0, wasm32-unknown-unknown)	2m 10s
build (stable, thumbv7em-none-eabi)	17s
test (stable)	16s
build (stable, wasm32-unknown-unknown)	11s
	7m 45s

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