[CI] Properly pass the cflags in the secp256k1 c++ test

Summary: The CFLAGS are set using a different environment in our codebase.

Test Plan:
Ran it on my own repo to check it fixes the build:
https://cirrus-ci.com/task/6380283690221568

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17004