[SECP256K1] Make scalar_inverse{,_var} benchmark scale with SECP256K1_BENCH_ITERS

Summary:
Completes backport of secp256k1#831:
https://github.com/bitcoin-core/secp256k1/pull/831/commits/24ad04fc064e71abdf973e061c30eb1f3f78db39

Depends on D9411.

Test Plan:

ninja bench-secp256k1

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9412