elliptic-curve: bump `crypto-bigint` to v0.6.0-pre.12 #966

Job	Run time
build (1.66.0, thumbv7em-none-eabi)	2m 19s
test (1.66.0)	2m 37s
build (1.66.0, wasm32-unknown-unknown)	2m 7s
test (stable)	44s
build (stable, thumbv7em-none-eabi)	8s
build (stable, wasm32-unknown-unknown)	8s
	8m 3s

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