[CI] Add a sepc256k1 specific build plan

Summary:
The secp256k1 CI tests are currently scattered all over the place. Add a
build that covers all the features in a single manageable place and
remove the duplicates.

Test Plan: Run the build-secp256k1 CI build.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5907