[CI] build libsecp256k1 for electrum tests

Summary:
Until now we tested electrum on CI using the pure-python fallback code of ecdsa.
libsecp256k1 is about to become a hard dependency for electrum, so build it for CI

Test Plan: run tests on CI, check that the python fallback code wasn't used (it would log a warning)

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16687