In ecc/bls12381, Pairing operations seems racy

[AnomalRoil]

It seems there are internal functions in circl that are "racy", because they are passing arrays using pointers, reading from these arrays and writing to these same arrays, causing data races when multiple go routines are operating on the same points at the same time.

This was discovered in the drand/kyber repo, where we have recently accepted a PR to add circl support for BLS12-381, but sadly despite having a test for data races in pairings, we didn't catch it earlier. (we weren't using -race in CI by mistake).

To reproduce:

git clone https://github.com/drand/kyber.git
cd kyber
go test -race ./pairing/circl_bls12381/...

This should fail in the TestRacePairings test.