Cuts go vet/go test wall time via a shared package-object cache.
- pre-commit and test steps move from docker.io/golang:1.26 to the
pinned estate gobuilder image (go1.26.7, trusts the internal CA)
- wire GOCACHEPROG to go-cache-plugin against the S3 cache bucket,
creds from org-level GOCACHE_AWS_* secrets
- bump test step memory limit to 4Gi for envtest + -race headroom
- buildx steps (build.yaml, docker.yaml) untouched: compilation
happens inside Dockerfile.operator, not the Woodpecker step