And also test gcc and clang snapshots.

secp256k1#1313

ci: Install development snapshots of gcc and clang

https://github.com/bitcoin-core/secp256k1/pull/1313/changes/1deecaaf3b94dbf08896e015e7f1e5ec328a40f2

ci: Add x86_64 Linux tasks for gcc and clang snapshots

https://github.com/bitcoin-core/secp256k1/pull/1313/changes/609093b3877b2fb21bd4bb2301a3eafb444a2fdb

secp256k1#1396

ci: Remove GCC build files and sage to reduce size of Docker image

ci, gha: Add "x86_64: Linux (Debian stable)" GitHub Actions job

ci: Remove "x86_64: Linux (Debian stable)" task from Cirrus CI

ci, gha: Add retry_builder Docker image builder

This change is aimed at significantly reducing the frequency of failures
caused by intermittent network timeouts.

ci, gha: Drop driver-opts.network input for setup-buildx-action

secp256k1#1719 (partial)

ci: Use YAML anchor and aliases for repeated "Checkout" steps

https://github.com/bitcoin-core/secp256k1/pull/1719/changes/574c2f3080eeb65995a1c1a430ed7f84803afd26

ci: Use YAML anchor and aliases for repeated "CI script" steps

https://github.com/bitcoin-core/secp256k1/pull/1719/changes/1decc49a1f08564064420468fc48b9aebd3989d3

Also initialize ecount in schnorrsig/test_impl.h to avoid a -Wuninitialized-const-pointer error with clang-snapshot. This will be done by core in https://github.com/bitcoin-core/secp256k1/pull/1186/changes/b19806122e9065c6f434fc6160cd0c57fa3fea8c

This is a backport of secp256k#1313, secp256k1#1396 and a partial backport of secp256k1#1719

push to github, check github actions run successfully