ci: Move "i686: Linux (Debian stable)" from Cirrus to GiHub Actions
ci: Move "s390x (big-endian): Linux..." from Cirrus to GitHub Actions
ci: Move "ARM32: Linux..." from Cirrus to GitHub Actions
ci: Move "ARM64: Linux..." from Cirrus to GitHub Actions
ci: Add "ppc64le: Linux..." to GitHub Actions
This is a backport of secp256k1#950 and secp256k1#1406
and some changes from https://github.com/bitcoin-core/secp256k1/pull/1719/changes/dff1bc107d593b12e824cbc04b0d220f2ffeb967 are needed to use *CI_SCRIPT_IN_DOCKER (which uses matrix.configuration.env_vars)
Depends on D19519