[SECP256K1] Add support for Cirrus CI

Summary:
Switch from Travis to Cirrus as a CI for libsecp256k1.

Backport of secp256k1#864

The files have been adapted to match our codebase:

• The build_autotools.sh and build_cmake.sh scripts are reused from the Travis setup.
• The nixos base file is removed since it would build openjdk from source for i686 and time out. This is not a good move but these files will be removed in a follow-up diff anyway, so not a big deal.
• Skip JNI tests on macOS, since Java is not installed on Cirrus macOS VM.
• Work around valgrind install path not being searched by CMake. This is a known CMake issue (/usr/local/include should be part of the system search path for headers): https://gitlab.kitware.com/cmake/cmake/-/issues/19120
• Increase the timeouts, since we are running each test twice the valgrind task will not complete in 1h on macOS.

Test Plan:
See result from my own repo:
https://cirrus-ci.com/build/6029790437179392

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D9397