[CI] Split secp256k1 benchmarks from the node benchmarks

Summary:
There is no reason to run the secp256k1 benchmarks in the same build as
the node benchmarks. Making it its own build will speed up the CI.

Test Plan:

../contrib/teamcity/build-configurations.py build-secp256k1-bench

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9228