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 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