elliptic-curve: bump `crypto-bigint` to v0.6.0-pre.12 #1848

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	2m 16s
minimal-versions	21s
build (1.65.0, wasm32-unknown-unknown)	2m 11s
build (stable, thumbv7em-none-eabi)	10s
build (stable, wasm32-unknown-unknown)	21s
test (1.65.0)	2m 15s
test (stable)	23s
	7m 57s

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