bench_bitcoin: Avoid read/write to default datadir

Summary:
Partial backport of Core PR13663
https://github.com/bitcoin/bitcoin/pull/13663/commits/fa43a4138b4a652bd83d70536539ae2aded05f0c

Test Plan:

make check
bench/bench_bitcoin

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: Fabien, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4686