[SECP256K1] ci: Switch all Linux builds to Debian and more improvements

Summary:
Backport of secp256k1#901

Main change is moving away from nix-shell to a regular debian docker.
The commit
https://github.com/bitcoin-core/secp256k1/pull/901/commits/c7f754fe4d5e032fd150c4b9b985855e9fcaa521
has not been ported since it is not relevant for us.

Depends on D9397.

Test Plan:
See result on my personal repo:
https://cirrus-ci.com/build/5828289764786176

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D9398