Call SHA256AutoDetect in benchmark setup
Call SHA256AutoDetect in benchmark setup

Summary:
> It seems SHA256AutoDetect() was not being called in benchmarks, making the numbers only reflect the naive implementation. Fix this by calling it in bench_bitcoin's setup.

This is a backport of [[https://github.com/bitcoin/bitcoin/pull/19214 | core#19214]]

Test Plan:
`ninja bench-bitcoin`

I don't see a significant difference in the SHA256 becnhmarks before or
after this commit.

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D10004

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