[BUILDBOT] Switch secp256k1 to Cirrus CI

Summary:
Since libsecp256k1 has been migrated from Travis to Cirrus CI, the build
panel is updated accordingly.

There is no equivalent to the Travis unit tests for Cirrus because the
logic is simpler, making a test not worth it.

Test Plan:

pytest

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D9416