Cut CI time/network by caching go build/test artifacts in S3.
- bump pre-commit/test steps to gobuilder:0.1.2-alma9
- wire GOCACHEPROG via go-cache-plugin with an absolute cache dir
- add GOCACHE_* env (ci-waitfordb prefix) and AWS creds from org secrets
- leave buildx build-check/docker steps untouched (compile is in Dockerfile)